Specifies an allowlist of files to include in the program. An error occurs if any of the files can’t be found.

json
{
  "compilerOptions": {},
  "files": [
    "core.ts",
    "sys.ts",
    "types.ts",
    "scanner.ts",
    "parser.ts",
    "utilities.ts",
    "binder.ts",
    "checker.ts",
    "tsc.ts"
  ]
}

This is useful when you only have a small number of files and don’t need to use a glob to reference many files. If you need that then use include.

Specifies an array of filenames or patterns to include in the program. These filenames are resolved relative to the directory containing the tsconfig.json file.

json
{
  "include": ["src/**/*", "tests/**/*"]
}

Which would include:

.
├── scripts                ⨯
│   ├── lint.ts            ⨯
│   ├── update_deps.ts     ⨯
│   └── utils.ts           ⨯
├── src                    ✓
│   ├── client             ✓
│   │    ├── index.ts      ✓
│   │    └── utils.ts      ✓
│   ├── server             ✓
│   │    └── index.ts      ✓
├── tests                  ✓
│   ├── app.test.ts        ✓
│   ├── utils.ts           ✓
│   └── tests.d.ts         ✓
├── package.json
├── tsconfig.json
└── yarn.lock

include and exclude support wildcard characters to make glob patterns:

• * matches zero or more characters (excluding directory separators)
• ? matches any one character (excluding directory separators)
• **/ matches any directory nested to any level

If a glob pattern doesn’t include a file extension, then only files with supported extensions are included (e.g. .ts, .tsx, and .d.ts by default, with .js and .jsx if allowJs is set to true).

Specifies an array of filenames or patterns that should be skipped when resolving include.

Important: exclude only changes which files are included as a result of the include setting. A file specified by exclude can still become part of your codebase due to an import statement in your code, a types inclusion, a /// <reference directive, or being specified in the files list.

It is not a mechanism that prevents a file from being included in the codebase - it simply changes what the include setting finds.

The value of extends is a string which contains a path to another configuration file to inherit from. The path may use Node.js style resolution.

The configuration from the base file are loaded first, then overridden by those in the inheriting config file. All relative paths found in the configuration file will be resolved relative to the configuration file they originated in.

It’s worth noting that files, include and exclude from the inheriting config file overwrite those from the base config file, and that circularity between configuration files is not allowed.

Example

configs/base.json:

json
{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

tsconfig.json:

json
{
  "extends": "./configs/base",
  "files": ["main.ts", "supplemental.ts"]
}

tsconfig.nostrictnull.json:

json
{
  "extends": "./tsconfig",
  "compilerOptions": {
    "strictNullChecks": false
  }
}

When you have a JavaScript project in your editor, TypeScript will provide types for your node_modules automatically using the DefinitelyTyped set of @types definitions. This is called automatic type acquisition, and you can customize it using the typeAcquisition object in your configuration.

If you would like to disable or customize this feature, create a jsconfig.json in the root of your project:

json
{
  "typeAcquisition": {
    "enable": false
  }
}

If you have a specific module which should be included (but isn’t in node_modules):

json
{
  "typeAcquisition": {
    "include": ["jest"]
  }
}

If a module should not be automatically acquired, for example if the library is available in your node_modules but your team has agreed to not use it:

json
{
  "typeAcquisition": {
    "exclude": ["jquery"]
  }
}

Project references are a way to structure your TypeScript programs into smaller pieces. Using Project References can greatly improve build and editor interaction times, enforce logical separation between components, and organize your code in new and improved ways.

You can read more about how references works in the Project References section of the handbook

Tells TypeScript to save information about the project graph from the last compilation to files stored on disk. This creates a series of .tsbuildinfo files in the same folder as your compilation output. They are not used by your JavaScript at runtime and can be safely deleted. You can read more about the flag in the 3.4 release notes.

To control which folders you want to the files to be built to, use the config option tsBuildInfoFile.

Modern browsers support all ES6 features, so ES6 is a good choice. You might choose to set a lower target if your code is deployed to older environments, or a higher target if your code is guaranteed to run in newer environments.

The target setting changes which JS features are downleveled and which are left intact. For example, an arrow function () => this will be turned into an equivalent function expression if target is ES5 or lower.

Changing target also changes the default value of lib. You may “mix and match” target and lib settings as desired, but you could just set target for convenience.

If your are only working with Node.js, here are recommended target based off of the Node version:

These are based on node.green’s database of support.

The special ESNext value refers to the highest version your TypeScript supports. This setting should be used with caution, since it doesn’t mean the same thing between different TypeScript versions and can make upgrades less predictable.

Sets the module system for the program. See the Modules chapter of the handbook for more information. You very likely want "CommonJS".

Here’s some example output for this file:

ts
// @filename: index.ts
import { valueOfPi } from "./constants";

export const twoPi = valueOfPi * 2;

CommonJS

js
const constants_1 = require("./constants");
exports.twoPi = constants_1.valueOfPi * 2;

UMD

js
(function (factory) {
    if (typeof module === "object" && typeof module.exports === "object") {
        var v = factory(require, exports);
        if (v !== undefined) module.exports = v;
    }
    else if (typeof define === "function" && define.amd) {
        define(["require", "exports", "./constants"], factory);
    }
})(function (require, exports) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    const constants_1 = require("./constants");
    exports.twoPi = constants_1.valueOfPi * 2;
});

AMD

js
define(["require", "exports", "./constants"], function (require, exports, constants_1) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.twoPi = constants_1.valueOfPi * 2;
});

System

js
System.register(["./constants"], function (exports_1, context_1) {
    "use strict";
    var constants_1, twoPi;
    var __moduleName = context_1 && context_1.id;
    return {
        setters: [
            function (constants_1_1) {
                constants_1 = constants_1_1;
            }
        ],
        execute: function () {
            exports_1("twoPi", twoPi = constants_1.valueOfPi * 2);
        }
    };
});

ESNext

js
import { valueOfPi } from "./constants";
export const twoPi = valueOfPi * 2;

TypeScript includes a default set of type definitions for built-in JS APIs (like Math), as well as type definitions for things found in browser environments (like document). TypeScript also includes APIs for newer JS features matching the target you specify; for example the definition for Map is available if target is ES6 or newer.

You may want to change these for a few reasons:

• Your program doesn’t run in a browser, so you don’t want the "dom" type definitions
• Your runtime platform provides certain JavaScript API objects (maybe through polyfills), but doesn’t yet support the full syntax of a given ECMAScript version
• You have polyfills or native implementations for some, but not all, of a higher level ECMAScript version

High Level libraries

Individual library components

This list may be out of date, you can see the full list in the TypeScript source code.

Allow JavaScript files to be imported inside your project, instead of just .ts and .tsx files. For example, this JS file:

js
// @filename: card.js
export const defaultCardDeck = "Heart";

When imported into a TypeScript file will raise an error:

ts
// @filename: index.ts
import { defaultCardDeck } from "./card";

console.log(defaultCardDeck);

Imports fine with allowJs enabled:

ts
// @filename: index.ts
import { defaultCardDeck } from "./card";

console.log(defaultCardDeck);

This flag can be used as a way to incrementally add TypeScript files into JS projects by allowing the .ts and .tsx files to live along-side existing JavaScript files.

Works in tandem with allowJs. When checkJs is enabled then errors are reported in JavaScript files. This is the equivalent of including // @ts-check at the top of all JavaScript files which are included in your project.

For example, this is incorrect JavaScript according to the parseFloat type definition which comes with TypeScript:

js
// parseFloat only takes a string
module.exports.pi = parseFloat(3.124);

When imported into a TypeScript module:

ts
// @filename: constants.js
module.exports.pi = parseFloat(3.124);

// @filename: index.ts
import { pi } from "./constants";
console.log(pi);

You will not get any errors. However, if you turn on checkJs then you will get error messages from the JavaScript file.

ts
// @filename: constants.js
Argument of type '3.124' is not assignable to parameter of type 'string'.2345Argument of type '3.124' is not assignable to parameter of type 'string'.module.exports.pi = parseFloat(3.124);

// @filename: index.ts
import { pi } from "./constants";
console.log(pi);

Controls how JSX constructs are emitted in JavaScript files. This only affects output of JS files that started in .tsx files.

• preserve: Emit .jsx files with the JSX unchanged
• react: Emit .js files with JSX changed to the equivalent React.createElement calls
• react-native: Emit .js files with the JSX unchanged

Generate d.ts files for every TypeScript or JavaScript file inside your project. These d.ts files are type definition files which describe the external API of your module. With d.ts files, tools like TypeScript can provide intellisense and accurate types for un-typed code.

When declaration is set to true, running the compiler with this TypeScript code:

ts
export let helloWorld let helloWorld: string= "hi";

Will generate an index.js file like this:

js
export let helloWorld = "hi";

With a corresponding helloWorld.d.ts:

ts
export declare let helloWorld: string;

When working with d.ts files for JavaScript files you may want to use emitDeclarationOnly or use outDir to ensure that the JavaScript files are not overwritten.

Generates a source map for .d.ts files which map back to the original .ts source file. This will allow editors such as VS Code to go to the original .ts file when using features like Go to Definition.

You should strongly consider turning this on if you’re using project references.

Enables the generation of sourcemap files. These files allow debuggers and other tools to display the original TypeScript source code when actually working with the emitted JavaScript files. Source map files are emitted as .js.map (or .jsx.map) files next to the corresponding .js output file.

The .js files will in turn contain a sourcemap comment to indicate to tools where the files are to external tools, for example:

ts
// helloWorld.ts
export declare const helloWorld = "hi";

Compiling with sourceMap set to true creates the following JavaScript file:

js
// helloWorld.js
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.helloWorld = "hi";
//# sourceMappingURL=// helloWorld.js.map

And this also generates this json map:

json
// helloWorld.js.map
{
  "version": 3,
  "file": "ex.js",
  "sourceRoot": "",
  "sources": ["../ex.ts"],
  "names": [],
  "mappings": ";;AAAa,QAAA,UAAU,GAAG,IAAI,CAAA"
}

If specified, all global (non-module) files will be concatenated into the single output file specified.

If module is system or amd, all module files will also be concatenated into this file after all global content.

Note: outFile cannot be used unless module is None, System, or AMD. This option cannot be used to bundle CommonJS or ES6 modules.

If specified, .js (as well as .d.ts, .js.map, etc.) files will be emitted into this directory. The directory structure of the original source files is preserved; see rootDir if the computed root is not what you intended.

If not specified, .js files will be emitted in the same directory as the .ts files they were generated from:

sh
$ tsc

example
├── index.js
└── index.ts

With a tsconfig.json like this:

json
{
  "compilerOptions": {
    "outDir": "dist"
  }
}

Running tsc with these settings moves the files into the specified dist folder:

sh
$ tsc

example
├── dist
│   └── index.js
├── index.ts
└── tsconfig.json

Default: The longest common path of all non-declaration input files. If composite is set, the default is instead the directory containing the tsconfig.json file.

When TypeScript compiles files, it keeps the same directory structure in the output directory as exists in the input directory.

For example, let’s say you have some input files:

MyProj
├── tsconfig.json
├── core
│   ├── a.ts
│   ├── b.ts
│   ├── sub
│   │   ├── c.ts
├── types.d.ts

The inferred value for rootDir is the longest common path of all non-declaration input files, which in this case is core/.

If your outDir was dist, TypeScript would write this tree:

MyProj
├── dist
│   ├── a.ts
│   ├── b.ts
│   ├── sub
│   │   ├── c.ts

However, you may have intended for core to be part of the output directory structure. By setting rootDir: "." in tsconfig.json, TypeScript would write this tree:

MyProj
├── dist
│   ├── core
│   │   ├── a.js
│   │   ├── b.js
│   │   ├── sub
│   │   │   ├── c.js

Importantly, rootDir does not affect which files become part of the compilation. It has no interaction with the include, exclude, or files tsconfig.json settings.

Note that TypeScript will never write an output file to a directory outside of outDir, and will never skip emitting a file. For this reason, rootDir also enforces that all files which need to be emitted are underneath the rootDir path.

For example, let’s say you had this tree:

MyProj
├── tsconfig.json
├── core
│   ├── a.ts
│   ├── b.ts
├── helpers.ts

It would be an error to specify rootDir as core and include as * because it creates a file (helpers.ts) that would need to be emitted outside the outDir (i.e. ../helpers.js).

The composite option enforces certain constraints which make it possible for build tools (including TypeScript itself, under --build mode) to quickly determine if a project has been built yet.

When this setting is on:

• The rootDir setting, if not explicitly set, defaults to the directory containing the tsconfig.json file.
• All implementation files must be matched by an include pattern or listed in the files array. If this constraint is violated, tsc will inform you which files weren’t specified.
• declaration defaults to true

You can find documentation on TypeScript projects in the handbook.

This setting lets you specify a file for storing incremental compilation information as a part of composite projects which enables faster building of larger TypeScript codebases. You can read more about composite projects in the handbook.

This option offers a way to configure the place where TypeScript keeps track of the files it stores on the disk to indicate a project’s build state — by default, they are in the same folder as your emitted JavaScript.

Strips all comments from TypeScript files when converting into JavaScript. Defaults to false.

For example, this is a TypeScript file which has a JSDoc comment:

ts
/** The translation of 'Hello world' into Portuguese */
export const helloWorldPTBR = "Olá Mundo";

When removeComments is set to true:

js
export const helloWorldPTBR = "Olá Mundo";

Without setting removeComments or having it as false:

js
/** The translation of 'Hello world' into Portuguese */
export const helloWorldPTBR = "Olá Mundo";

This means that your comments will show up in the JavaScript code.

Do not emit compiler output files like JavaScript source code, source-maps or declarations.

This makes room for another tool like Babel, or swc to handle converting the TypeScript file to a file which can run inside a JavaScript environment.

You can then use TypeScript as a tool for providing editor integration, and as a source code type-checker.

For certain downleveling operations, TypeScript uses some helper code for operations like extending class, spreading arrays or objects, and async operations. By default, these helpers are inserted into files which use them. This can result in code duplication if the same helper is used in many different modules.

If the importHelpers flag is on, these helper functions are instead imported from the tslib module. You will need to ensure that the tslib module is able to be imported at runtime. This only affects modules; global script files will not attempt to import modules.

For example, with this TypeScript:

ts
export function fn(arr: number[]) {
  const arr2 = [1, ...arr];
}

Turning on downlevelIteration and importHelpers is still false:

js
var __read = (this && this.__read) || function (o, n) {
    var m = typeof Symbol === "function" && o[Symbol.iterator];
    if (!m) return o;
    var i = m.call(o), r, ar = [], e;
    try {
        while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);
    }
    catch (error) { e = { error: error }; }
    finally {
        try {
            if (r && !r.done && (m = i["return"])) m.call(i);
        }
        finally { if (e) throw e.error; }
    }
    return ar;
};
var __spread = (this && this.__spread) || function () {
    for (var ar = [], i = 0; i < arguments.length; i++) ar = ar.concat(__read(arguments[i]));
    return ar;
};
export function fn(arr) {
    var arr2 = __spread([1], arr);
}

Then turning on both downlevelIteration and importHelpers:

js
import { __read, __spread } from "tslib";
export function fn(arr) {
    var arr2 = __spread([1], arr);
}

You can use noEmitHelpers when you provide your own implementations of these functions.

Downleveling is TypeScript’s term for transpiling to an older version of JavaScript. This flag is to enable support for a more accurate implementation of how modern JavaScript iterates through new concepts in older JavaScript runtimes.

ECMAScript 6 added several new iteration primitives: the for / of loop (for (el of arr)), Array spread ([a, ...b]), argument spread (fn(...args)), and Symbol.iterator. --downlevelIteration allows for these iteration primitives to be used more accurately in ES5 environments if a Symbol.iterator implementation is present.

Example: Effects on for / of

Without downlevelIteration on, a for / of loop on any object is downleveled to a traditional for loop:

js
"use strict";
var str = "Hello!";
for (var _i = 0, str_1 = str; _i < str_1.length; _i++) {
    var s = str_1[_i];
    console.log(s);
}

This is often what people expect, but it’s not 100% compliant with ECMAScript 6 behavior. Certain strings, such as emoji (😜), have a .length of 2 (or even more!), but should iterate as 1 unit in a for-of loop. See this blog post by Jonathan New for a longer explanation.

When downlevelIteration is enabled, TypeScript will use a helper function that checks for a Symbol.iterator implementation (either native or polyfill). If this implementation is missing, you’ll fall back to index-based iteration.

js
"use strict";
var __values = (this && this.__values) || function(o) {
    var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;
    if (m) return m.call(o);
    if (o && typeof o.length === "number") return {
        next: function () {
            if (o && i >= o.length) o = void 0;
            return { value: o && o[i++], done: !o };
        }
    };
    throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");
};
var e_1, _a;
var str = "Hello!";
try {
    for (var str_1 = __values(str), str_1_1 = str_1.next(); !str_1_1.done; str_1_1 = str_1.next()) {
        var s = str_1_1.value;
        console.log(s);
    }
}
catch (e_1_1) { e_1 = { error: e_1_1 }; }
finally {
    try {
        if (str_1_1 && !str_1_1.done && (_a = str_1.return)) _a.call(str_1);
    }
    finally { if (e_1) throw e_1.error; }
}

Note: enabling downlevelIteration does not improve compliance if Symbol.iterator is not present in the runtime.

Example: Effects on Array Spreads

This is an array spread:

js
// Make a new array who elements are 1 followed by the elements of arr2
const arr = [1, ...arr2];

Based on the description, it sounds easy to downlevel to ES5:

js
// The same, right?
const arr = [1].concat(arr2);

However, this is observably different in certain rare cases. For example, if an array has a “hole” in it, the missing index will create an own property if spreaded, but will not if built using concat:

js
// Make an array where the '1' element is missing
let missing = [0, , 1];
let spreaded = [...missing];
let concated = [].concat(missing);

// true
"1" in spreaded;
// false
"1" in concated;

Just as with for / of, downlevelIteration will use Symbol.iterator (if present) to more accurately emulate ES 6 behavior.

While you can use TypeScript to produce JavaScript code from TypeScript code, it’s also common to use other transpilers such as Babel to do this. However, other transpilers only operate on a single file at a time, which means they can’t apply code transforms that depend on understanding the full type system. This restriction also applies to TypeScript’s ts.transpileModule API which is used by some build tools.

These limitations can cause runtime problems with some TypeScript features like const enums and namespaces. Setting the isolatedModules flag tells TypeScript to warn you if you write certain code that can’t be correctly interpreted by a single-file transpilation process.

It does not change the behavior of your code, or otherwise change the behavior of TypeScript’s checking and emitting process.

Some examples of code which does not work when isolatedModules is enabled.

Exports of Non-Value Identifiers

In TypeScript, you can import a type and then subsequently export it:

ts
import { someType,import someType someFunction import someFunction} from "someModule";

someFunction(import someFunction);

export { someType,export someType someFunction export someFunction};

Because there’s no value for someType, the emitted export will not try to export it (this would be a runtime error in JavaScript):

js
export { someFunction };

Single-file transpilers don’t know whether someType produces a value or not, so it’s an error to export a name that only refers to a type.

Non-Module Files

If isolatedModules is set, all implementation files must be modules (which means it has some form of import/export). An error occurs if any file isn’t a module:

ts
 fn(function fn(): void) {}
All files must be modules when the '--isolatedModules' flag is provided.1208All files must be modules when the '--isolatedModules' flag is provided.

This restriction doesn’t apply to .d.ts files

References to const enum members

In TypeScript, when you reference a const enum member, the reference is replaced by its actual value in the emitted JavaScript. Changing this TypeScript:

ts
declare const enum Numbers const enum Numbers
  Zero (enum member) Numbers.Zero = 0= 0,
  One (enum member) Numbers.One = 1= 1
}
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidNumbers.const enum NumbersZero (enum member) Numbers.Zero = 0+ Numbers.const enum NumbersOne)(enum member) Numbers.One = 1

To this JavaScript:

js
"use strict";
console.log(0 + 1);

Without knowledge of the values of these members, other transpilers can’t replace the references to Number, which would be a runtime error if left alone (since there are no Numbers object at runtime). Because of this, when isolatedModules is set, it is an error to reference an ambient const enum member.

The strict flag enables a wide range of type checking behavior that results in stronger guarantees of program correctness. Turning this on is equivalent to enabling all of the strict mode family options, which are outlined below. You can then turn off individual strict mode family checks as needed.

Future versions of TypeScript may introduce additional stricter checking under this flag, so upgrades of TypeScript might result in new type errors in your program. When appropriate and possible, a corresponding flag will be added to disable that behavior.

In some cases where no type annotations are present, TypeScript will fall back to a type of any for a variable when it cannot infer the type.

This can cause some errors to be missed, for example:

ts
function fn(function fn(s: any): voids)(parameter) s: any {
  // No error?
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voids.(parameter) s: anysubtr(any3));
}
fn(function fn(s: any): void42);

Turning on noImplicitAny however TypeScript will issue an error whenever it would have inferred any:

ts
function fn(function fn(s: any): voids)(parameter) s: any {
Parameter 's' implicitly has an 'any' type.7006Parameter 's' implicitly has an 'any' type.  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voids.(parameter) s: anysubtr(any3));
}

When strictNullChecks is false, null and undefined are effectively ignored by the language. This can lead to unexpected errors at runtime.

When strictNullChecks is true, null and undefined have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.

For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will:

ts
declare const loggedInUsername:const loggedInUsername: string string;

const users const users: {
    name: string;
    age: number;
}[]= [
  { name:(property) name: string"Oby", age:(property) age: number12 },
  { name:(property) name: string"Heera", age:(property) age: number32 }
];

const loggedInUser const loggedInUser: {
    name: string;
    age: number;
}= users.const users: {
    name: string;
    age: number;
}[]find((method) Array<{ name: string; age: number; }>.find(predicate: (value: {
    name: string;
    age: number;
}, index: number, obj: {
    name: string;
    age: number;
}[]) => unknown, thisArg?: any): {
    name: string;
    age: number;
} (+1 overload)u (parameter) u: {
    name: string;
    age: number;
}=> u.(parameter) u: {
    name: string;
    age: number;
}name (property) name: string=== loggedInUsername)const loggedInUsername: string
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidloggedInUser.const loggedInUser: {
    name: string;
    age: number;
}age)(property) age: number

Setting strictNullChecks to true will raise an error that you have not made a guarantee that the loggedInUser exists before trying to use it.

ts
declare const loggedInUsername:const loggedInUsername: string string;

const users const users: {
    name: string;
    age: number;
}[]= [
  { name:(property) name: string"Oby", age:(property) age: number12 },
  { name:(property) name: string"Heera", age:(property) age: number32 }
];

const loggedInUser const loggedInUser: {
    name: string;
    age: number;
} | undefined= users.const users: {
    name: string;
    age: number;
}[]find((method) Array<{ name: string; age: number; }>.find(predicate: (value: {
    name: string;
    age: number;
}, index: number, obj: {
    name: string;
    age: number;
}[]) => unknown, thisArg?: any): {
    name: string;
    age: number;
} | undefined (+1 overload)u (parameter) u: {
    name: string;
    age: number;
}=> u.(parameter) u: {
    name: string;
    age: number;
}name (property) name: string=== loggedInUsername)const loggedInUsername: string
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidloggedInUser.const loggedInUser: {
    name: string;
    age: number;
} | undefinedage)(property) age: number
Object is possibly 'undefined'.2532Object is possibly 'undefined'.

The second example failed because the array’s find function looks a bit like this simplification:

ts
// When strictNullChecks: true
type Array = {
  find(predicate: (value: any, index: number) => boolean): S | undefined;
};

// When strictNullChecks: false the undefined is removed from the type system,
// allowing you to write code which assumes it always found a result
type Array = {
  find(predicate: (value: any, index: number) => boolean): S;
};

When enabled, this flag causes functions parameters to be checked more correctly.

Here’s a basic example with strictFunctionTypes off:

ts
function fn(function fn(x: string): voidx:(parameter) x: string string) {
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"Hello, " + x.(parameter) x: stringtoLowerCase((method) String.toLowerCase(): string));
}

type StringOrNumberFunc type StringOrNumberFunc = (ns: string | number) => void= (ns:(parameter) ns: string | number string | number) => void;

// Unsafe assignment
let func:let func: StringOrNumberFunc StringOrNumberFunc type StringOrNumberFunc = (ns: string | number) => void= fn;function fn(x: string): void
// Unsafe call - will crash
func(let func: (ns: string | number) => void10);

With strictFunctionTypes on, the error is correctly detected:

ts
function fn(function fn(x: string): voidx:(parameter) x: string string) {
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"Hello, " + x.(parameter) x: stringtoLowerCase((method) String.toLowerCase(): string));
}

type StringOrNumberFunc type StringOrNumberFunc = (ns: string | number) => void= (ns:(parameter) ns: string | number string | number) => void;

// Unsafe assignment is prevented
let func:let func: StringOrNumberFunc StringOrNumberFunc type StringOrNumberFunc = (ns: string | number) => void= fn;function fn(x: string): void
Type '(x: string) => void' is not assignable to type 'StringOrNumberFunc'.
  Types of parameters 'x' and 'ns' are incompatible.
    Type 'string | number' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'.2322Type '(x: string) => void' is not assignable to type 'StringOrNumberFunc'.
  Types of parameters 'x' and 'ns' are incompatible.
    Type 'string | number' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'.

During development of this feature, we discovered a large number of inherently unsafe class hierarchies, including some in the DOM. Because of this, the setting only applies to functions written in function syntax, not to those in method syntax:

ts
type Methodish type Methodish = {
    func(x: string | number): void;
}= {
  func((method) func(x: string | number): voidx:(parameter) x: string | number string | number): void;
};

function fn(function fn(x: string): voidx:(parameter) x: string string) {
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"Hello, " + x.(parameter) x: stringtoLowerCase((method) String.toLowerCase(): string));
}

// Ultimately an unsafe assignment, but not detected
const m:const m: Methodish Methodish type Methodish = {
    func(x: string | number): void;
}= {
  func:(method) func(x: string | number): void fnfunction fn(x: string): void
};
m.const m: Methodishfunc((method) func(x: string | number): void10);

When set, TypeScript will check that the built-in methods of functions call, bind, and apply are invoked with correct argument for the underlying function:

ts
// With strictBindCallApply on
function fn(function fn(x: string): numberx:(parameter) x: string string) {
  return parseInt(function parseInt(s: string, radix?: number | undefined): numberx)(parameter) x: string
}

const n1 const n1: number= fn.function fn(x: string): numbercall((method) CallableFunction.call<undefined, [string], number>(this: (this: undefined, args_0: string) => number, thisArg: undefined, args_0: string): numberundefinedvar undefined, "10");

const n2 const n2: number= fn.function fn(x: string): numbercall((method) CallableFunction.call<undefined, [string], number>(this: (this: undefined, x: string) => number, thisArg: undefined, x: string): numberundefinedvar undefined, false);
Argument of type 'false' is not assignable to parameter of type 'string'.2345Argument of type 'false' is not assignable to parameter of type 'string'.

Otherwise, these functions accept any arguments and will return any:

ts
// With strictBindCallApply off
function fn(function fn(x: string): numberx:(parameter) x: string string) {
  return parseInt(function parseInt(s: string, radix?: number | undefined): numberx)(parameter) x: string
}

// Note: No error; return type is 'any'
const n const n: any= fn.function fn(x: string): numbercall((method) Function.call(this: Function, thisArg: any, ...argArray: any[]): anyundefinedvar undefined, false);

When set to true, TypeScript will raise an error when a class property was declared but not set in the constructor.

ts
class UserAccount class UserAccount
  name:(property) UserAccount.name: string string;
  accountType (property) UserAccount.accountType: string= "user";

  email:(property) UserAccount.email: string string;
Property 'email' has no initializer and is not definitely assigned in the constructor.2564Property 'email' has no initializer and is not definitely assigned in the constructor.  address:(property) UserAccount.address: string | undefined string | undefined;

  constructor(name:(parameter) name: string string) {
    this.name (property) UserAccount.name: string= name;(parameter) name: string
    // Note that this.email is not set
  }
}

In the above case:

• this.name is set specifically.
• this.accountType is set by default.
• this.email is not set and raises an error.
• this.address is declared as potentially undefined which means it does not have to be set.

Raise error on ‘this’ expressions with an implied ‘any’ type.

For example, the class below returns a function which tries to access this.width and this.height – but the context for this inside the function inside getAreaFunction is not the instance of the Rectangle.

ts
class Rectangle class Rectangle
  width:(property) Rectangle.width: number number;
  height:(property) Rectangle.height: number number;

  constructor(width:(parameter) width: number number, height:(parameter) height: number number) {
    this.width (property) Rectangle.width: number= width;(parameter) width: number
    this.height (property) Rectangle.height: number= height;(parameter) height: number
  }

  getAreaFunction((method) Rectangle.getAreaFunction(): () => number) {
    return function() {
      return this.width any* this.height;any
'this' implicitly has type 'any' because it does not have a type annotation.
'this' implicitly has type 'any' because it does not have a type annotation.2683
2683'this' implicitly has type 'any' because it does not have a type annotation.
'this' implicitly has type 'any' because it does not have a type annotation.    };
  }
}

Ensures that your files are parsed in the ECMAScript strict mode, and emit “use strict” for each source file.

ECMAScript strict mode was introduced in ES5 and provides behavior tweaks to the runtime of the JavaScript engine to improve performance, and makes a set of errors throw instead of silently ignoring them.

Specify module resolution strategy: ‘node’ (Node.js) or ‘classic’ (TypeScript pre-1.6). You probably won’t need to use this.

Lets you set a base directory to resolve non-absolute module names.

You can define a root folder where you can do absolute file resolution. E.g.

baseUrl
├── ex.ts
├── hello
│   └── world.ts
└── tsconfig.json

With "baseUrl": "./" inside this project TypeScript will look for files starting at the same folder as the tsconfig.json.

ts
import { helloWorld } from "hello/world";

console.log(helloWorld);

If you get tired of imports always looking like "../" or "./". Or needing to change as you move files, this is a great way to fix that.

A series of entries which re-map imports to lookup locations relative to the baseUrl, there is a larger coverage of paths in the handbook.

paths lets you declare how TypeScript should resolve an import in your require/imports.

json
{
  "compilerOptions": {
    "baseUrl": ".", // this must be specified if "paths" is specified.
    "paths": {
      "jquery": ["node_modules/jquery/dist/jquery"] // this mapping is relative to "baseUrl"
    }
  }
}

This would allow you to be able to write import "jquery", and get all of the correct typing locally.

json
{
  "compilerOptions": {
    "baseUrl": "src",
    "paths": {
        "app/*": ["app/*"],
        "config/*": ["app/_config/*"],
        "environment/*": ["environments/*"],
        "shared/*": ["app/_shared/*"],
        "helpers/*": ["helpers/*"],
        "tests/*": ["tests/*"]
    },
}

In this case, you can tell the TypeScript file resolver to support a number of custom prefixes to find code. This pattern can be used to avoid long relative paths within your codebase.

Using rootDirs, you can inform the compiler that there are many “virtual” directories acting as a single root. This allows the compiler to resolve relative module imports within these “virtual” directories, as if they were merged in to one directory.

For example:

 src
 └── views
     └── view1.ts (can import "./template1", "./view2`)
     └── view2.ts (can import "./template1", "./view1`)

 generated
 └── templates
         └── views
             └── template1.ts (can import "./view1", "./view2")

json
{
  "compilerOptions": {
    "rootDirs": ["src/views", "generated/templates/views"]
  }
}

This does not affect how TypeScript emits JavaScript, it only emulates the assumption that they will be able to work via those relative paths at runtime.

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If typeRoots is specified, only packages under typeRoots will be included. For example:

json
{
  "compilerOptions": {
    "typeRoots": ["./typings", "./vendor/types"]
  }
}

This config file will include all packages under ./typings and ./vendor/types, and no packages from ./node_modules/@types. All paths are relative to the tsconfig.json.

By default all visible ”@types” packages are included in your compilation. Packages in node_modules/@types of any enclosing folder are considered visible. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on.

If types is specified, only packages listed will be included. For instance:

json
{
  "compilerOptions": {
    "types": ["node", "lodash", "express"]
  }
}

This tsconfig.json file will only include ./node_modules/@types/node, ./node_modules/@types/lodash and ./node_modules/@types/express. Other packages under node_modules/@types/* will not be included.

This feature differs from typeRoots in that it is about specifying only the exact types you want included, whereas typeRoots supports saying you want particular folders.

When set to true, allowSyntheticDefaultImports let’s you write an import like:

ts
import React from "react";

instead of:

ts
import * as React from "react";

When the module does not specify a default export.

This does not affect the JavaScript emitted by TypeScript, it only for the type checking. This option brings the behavior of TypeScript in-line with Babel, where extra code is emitted to make using a default export of a module more ergonomic.

Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports.

TypeScript adheres to the EcmaScript standard for modules, which means that a file with exports would have to specifically include a default export in order to support syntax like import React from "react". This export pattern is rare in modules for CommonJS. For example, without esModuleInterop as true:

ts
// @filename: utilFunctions.js
const getStringLength = str => str.length;

ports =anymodule.module export=
(property) export=: {
    getStringLength: (str: any) => any;
}exports = {
StringLemodule export=
(property) export=: {
    getStringLength: (str: any) => any;
}ngth
};

// @filename: index.ts
import utils from "./utilFunctions";

const count = utils.getStringLength("Check JS");

This won’t work because there isn’t a default object which you can import. Even though it feels like it should. For convenience, transpilers like Babel will automatically create a default if one isn’t created. Making the module look a bit more like:

js
// @filename: utilFunctions.js
const getStringLength = str => str.length;
const allFunctions = {
  getStringLength
};

module.exports = allFunctions;

Turning on this compiler flag will also enable allowSyntheticDefaultImports.

This is to reflect the same flag in Node.js; which does not resolve the real path of symlinks.

This flag also exhibits the opposite behavior to Webpack’s resolve.symlinks option (i.e. setting TypeScript’s preserveSymlinks to true parallels setting Webpack’s resolve.symlinks to false, and vice-versa).

With this enabled, references to modules and packages (e.g. imports and /// <reference type="..." /> directives) are all resolved relative to the location of the symbolic link file, rather than relative to the path that the symbolic link resolves to.

When set to true, allowUmdGlobalAccess lets you access UMD exports as globals from inside module files. A module file is a file that has imports and/or exports. Without this flag, using an export from a UMD module requires an import declaration.

An example use case for this flag would be a web project where you know the particular library (like jQuery or Lodash) will always be available at runtime, but you can’t access it with an import.

Specify the location where a debugger should locate TypeScript files instead of relative source locations. This string is treated verbatim inside the source-map where you can use a path or a URL:

json
{
  "compilerOptions": {
    "sourceMap": true,
    "sourceRoot": "https://my-website.com/debug/source/"
  }
}

Would declare that index.js will have a source file at https://my-website.com/debug/source/index.ts.

Specify the location where debugger should locate map files instead of generated locations. This string is treated verbatim inside the source-map, for example:

json
{
  "compilerOptions": {
    "sourceMap": true,
    "mapRoot": "https://my-website.com/debug/sourcemaps/"
  }
}

Would declare that index.js will have sourcemaps at https://my-website.com/debug/sourcemaps/index.js.map.

When set, instead of writing out a .js.map file to provide source maps, TypeScript will embed the source map content in the .js files. Although this results in larger JS files, it can be convenient in some scenarios. For example, you might want to debug JS files on a webserver that doesn’t allow .map files to be served.

Mutually exclusive with sourceMap.

For example, with this TypeScript:

ts
const helloWorld = "hi";
console.log(helloWorld);

Converts to this JavaScript:

js
"use strict";
const helloWorld = "hi";
console.log(helloWorld);

Then enable building it with inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file.

js
"use strict";
const helloWorld = "hi";
console.log(helloWorld);
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMifQ==

When set, TypeScript will include the original content of the .ts file as an embedded string in the source map. This is often useful in the same cases as inlineSourceMap.

Requires either sourceMap or inlineSourceMap to be set.

For example, with this TypeScript:

ts
const helloWorld const helloWorld: "hi"= "hi";
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidhelloWorld)const helloWorld: "hi"

By default converts to this JavaScript:

js
"use strict";
const helloWorld = "hi";
console.log(helloWorld);

Then enable building it with inlineSources and inlineSourceMap enabled there is a comment at the bottom of the file which includes a source-map for the file. Note that the end is different from the example in inlineSourceMap because the source-map now contains the original source code also.

js
"use strict";
const helloWorld = "hi";
console.log(helloWorld);
//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJmaWxlIjoiaW5kZXguanMiLCJzb3VyY2VSb290IjoiIiwic291cmNlcyI6WyJpbmRleC50cyJdLCJuYW1lcyI6W10sIm1hcHBpbmdzIjoiO0FBQUEsTUFBTSxVQUFVLEdBQUcsSUFBSSxDQUFDO0FBQ3hCLE9BQU8sQ0FBQyxHQUFHLENBQUMsVUFBVSxDQUFDLENBQUMiLCJzb3VyY2VzQ29udGVudCI6WyJjb25zdCBoZWxsb1dvcmxkID0gXCJoaVwiO1xuY29uc29sZS5sb2coaGVsbG9Xb3JsZCk7Il19

Report errors on unused local variables.

ts
const createKeyboard const createKeyboard: (modelID: number) => {
    type: string;
    modelID: number;
}= (modelID:(parameter) modelID: number number) => {
  const defaultModelID const defaultModelID: 23= 23;
'defaultModelID' is declared but its value is never read.6133'defaultModelID' is declared but its value is never read.  return { type:(property) type: string"keyboard", modelID (property) modelID: number};
};

Report errors on unused parameters in functions.

ts
const createDefaultKeyboard const createDefaultKeyboard: (modelID: number) => {
    type: string;
    modelID: number;
}= (modelID:(parameter) modelID: number number) => {
'modelID' is declared but its value is never read.6133'modelID' is declared but its value is never read.  const defaultModelID const defaultModelID: 23= 23;
  return { type:(property) type: string"keyboard", modelID:(property) modelID: number defaultModelID const defaultModelID: 23};
};

When enabled, TypeScript will check all code paths in a function to ensure they return a value.

ts
function lookupHeadphonesManufacturer(function lookupHeadphonesManufacturer(color: "blue" | "black"): stringcolor:(parameter) color: "blue" | "black""blue" | "black"): string 
Function lacks ending return statement and return type does not include 'undefined'.2366Function lacks ending return statement and return type does not include 'undefined'.  if (color (parameter) color: "blue" | "black"=== "blue") {
    return "beats";
  } else {
    "bose";
  }
}

Report errors for fallthrough cases in switch statements. Ensures that any non-empty case inside a switch statement includes either break or return. This means you won’t accidentally ship a case fallthrough bug.

ts
const a:const a: number number = 6;

switch (a)const a: number {
  case 0:
Fallthrough case in switch.7029Fallthrough case in switch.    console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"even");
  case 1:
    console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"odd");
    break;
}

Enables experimental support for decorators, which is in stage 2 of the TC39 standardization process.

Decorators are a language feature which hasn’t yet been fully ratified into the JavaScript specification. This means that the implementation version in TypeScript may differ from the implementation in JavaScript when it it decided by TC39.

You can find out more about decorator support in TypeScript in the handbook.

Enables experimental support for emitting type metadata for decorators which works with the module reflect-metadata.

For example, here is the JavaScript

ts
function LogMethod(function LogMethod(target: any, propertyKey: string | symbol, descriptor: PropertyDescriptor): void
  target:(parameter) target: any any,
  propertyKey:(parameter) propertyKey: string | symbol string | symbol,
  descriptor:(parameter) descriptor: PropertyDescriptor PropertyDescriptorinterface PropertyDescriptor
) {
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidtarget)(parameter) target: any
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidpropertyKey)(parameter) propertyKey: string | symbol
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voiddescriptor)(parameter) descriptor: PropertyDescriptor
}

class Demo class Demo
  @LogMethodfunction LogMethod(target: any, propertyKey: string | symbol, descriptor: PropertyDescriptor): void
  public foo((method) Demo.foo(bar: number): voidbar:(parameter) bar: number number) {
    // do nothing
  }
}

const demo const demo: Demo= new Demo(constructor Demo(): Demo);

With emitDecoratorMetadata not set to true (default):

js
"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
function LogMethod(target, propertyKey, descriptor) {
    console.log(target);
    console.log(propertyKey);
    console.log(descriptor);
}
class Demo {
    foo(bar) {
        // do nothing
    }
}
__decorate([
    LogMethod
], Demo.prototype, "foo", null);
const demo = new Demo();

With emitDecoratorMetadata set to true:

js
"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function LogMethod(target, propertyKey, descriptor) {
    console.log(target);
    console.log(propertyKey);
    console.log(descriptor);
}
class Demo {
    foo(bar) {
        // do nothing
    }
}
__decorate([
    LogMethod,
    __metadata("design:type", Function),
    __metadata("design:paramtypes", [Number]),
    __metadata("design:returntype", void 0)
], Demo.prototype, "foo", null);
const demo = new Demo();

For certain downleveling operations, TypeScript uses some helper code for operations like extending class, spreading arrays or objects, and async operations. By default, these helpers are inserted into files which use them. This can result in code duplication if the same helper is used in many different modules.

If the importHelpers flag is on, these helper functions are instead imported from the tslib module. You will need to ensure that the tslib module is able to be imported at runtime. This only affects modules; global script files will not attempt to import modules.

For example, with this TypeScript:

ts
export function fn(arr: number[]) {
  const arr2 = [1, ...arr];
}

Turning on downlevelIteration and importHelpers is still false:

js
var __read = (this && this.__read) || function (o, n) {
    var m = typeof Symbol === "function" && o[Symbol.iterator];
    if (!m) return o;
    var i = m.call(o), r, ar = [], e;
    try {
        while ((n === void 0 || n-- > 0) && !(r = i.next()).done) ar.push(r.value);
    }
    catch (error) { e = { error: error }; }
    finally {
        try {
            if (r && !r.done && (m = i["return"])) m.call(i);
        }
        finally { if (e) throw e.error; }
    }
    return ar;
};
var __spread = (this && this.__spread) || function () {
    for (var ar = [], i = 0; i < arguments.length; i++) ar = ar.concat(__read(arguments[i]));
    return ar;
};
export function fn(arr) {
    var arr2 = __spread([1], arr);
}

Then turning on both downlevelIteration and importHelpers:

js
import { __read, __spread } from "tslib";
export function fn(arr) {
    var arr2 = __spread([1], arr);
}

You can use noEmitHelpers when you provide your own implementations of these functions.

Print names of files part of the compilation. This is useful when you are not sure that TypeScript has included a file you expected.

For example:

example
├── index.ts
├── package.json
└── tsconfig.json

With:

json
{
  "compilerOptions": {
    "listFiles": true
  }
}

Would echo paths like:

$ npm run tsc
path/to/example/node_modules/typescript/lib/lib.d.ts
path/to/example/node_modules/typescript/lib/lib.es5.d.ts
path/to/example/node_modules/typescript/lib/lib.dom.d.ts
path/to/example/node_modules/typescript/lib/lib.webworker.importscripts.d.ts
path/to/example/node_modules/typescript/lib/lib.scripthost.d.ts
path/to/example/index.ts

Print names of generated files part of the compilation to the terminal.

This flag is useful in two cases:

• You want to transpile TypeScript as a part of a build chain in the terminal where the filenames are processed in the next command.
• You are not sure that TypeScript has included a file you expected, as a part of debugging the file inclusion settings.

For example:

example
├── index.ts
├── package.json
└── tsconfig.json

With:

json
{
  "compilerOptions": {
    "declaration": true,
    "listFiles": true
  }
}

Would echo paths like:

$ npm run tsc

path/to/example/index.js
path/to/example/index.d.ts

Normally, TypeScript would return silently on success.

When you are trying to debug why a module isn’t being included. You can set traceResolutions to true to have TypeScript print information about its resolution process for each processed file.

You can read more about this in the handbook.

Used to output diagnostic information for debugging. This command is a subset of extendedDiagnostics which are more user-facing results, and easier to interpret.

If you have been asked by a TypeScript compiler engineer to give the results using this flag in a compile, in which there is no harm in using --extendedDiagnostics instead.

You can use this flag to discover where TypeScript is spending it’s time when compiling. This is a tool used for understanding the performance characteristics of your codebase overall.

You can learn more about how to measure and understand the output in the performance section of the wiki.

This option gives you the chance to have TypeScript emit a v8 CPU profile during the compiler run. The CPU profile can provide insight into why your builds may be slow.

This option can only be used from the CLI via: --generateCpuProfile tsc-output.cpuprofile.

sh
npm run tsc --generateCpuProfile tsc-output.cpuprofile

This file can be opened in a chromium based browser like Chrome or Edge Developer in the CPU profiler section. You can learn more about understanding the compilers performance in the TypeScript wiki section on performance.

When this option is enabled, TypeScript will avoid rechecking/rebuilding all truly possibly-affected files, and only recheck/rebuild files that have changed as well as files that directly import them.

This can be considered a ‘fast & loose’ implementation of the watching algorithm, which can drastically reduce incremental rebuild times at the expense of having to run the full build occasionally to get all compiler error messages.

Only emit .d.ts files; do not emit .js files.

This setting is useful in two cases:

• You are using a transpiler other than TypeScript to generate your JavaScript.
• You are using TypeScript to only generate d.ts files for your consumers.

This flag controls how import works, there are 3 different options:

• remove: The default behavior of dropping import statements which only reference types.
• preserve: Preserves all import statements whose values or types are never used. This can cause imports/side-effects to be preserved.
• error: This preserves all imports (the same as the preserve option), but will error when a value import is only used as a type. This might be useful if you want to ensure no values are being accidentally imported, but still make side-effect imports explicit.

This flag works because you can use import type to explicitly create an import statement which should never be emitted into JavaScript.

Changes the function called in .js files when compiling JSX Elements. The most common change is to use "h" or "preact.h" instead of the default "React.createElement" if using preact.

This is the same as Babel’s /** @jsx h */ directive.

Allows importing modules with a ‘.json’ extension, which is a common practice in node projects. This includes generating a type for the import based on the static JSON shape.

TypeScript does not support resolving JSON files by default:

ts
// @filename: settings.json
{
    "repo": "TypeScript",
    "dry": false,
    "debug": false
}
// @filename: index.ts
import settings from "./settings.json";

settings.debug === true;
settings.dry === 2;

Enabling the option allows importing JSON, and validating the types in that JSON file.

ts
// @filename: settings.json
{
    "repo": "TypeScript",
    "dry": false,
    "debug": false
}
// @filename: index.ts
import settings from "./settings.json";

settings.debug === true;
settings.dry === 2;

Use outFile instead.

The out option computes the final file location in a way that is not predictable or consistent. This option is retained for backward compatibility only and is deprecated.

Use --jsxFactory instead. Specify the object invoked for createElement when targeting react for TSX files.

Use --skipLibCheck instead. Skip type checking of default library declaration files.

In prior versions of TypeScript, this controlled what encoding was used when reading text files from disk. Today, TypeScript assumes UTF-8 encoding, but will correctly detect UTF-16 (BE and LE) or UTF-8 BOMs.

Controls whether TypeScript will emit a byte order mark (BOM) when writing output files. Some runtime environments require a BOM to correctly interpret a JavaScript files; others require that it is not present. The default value of false is generally best unless you have a reason to change it.

Specify the end of line sequence to be used when emitting files: ‘CRLF’ (dos) or ‘LF’ (unix).

Do not truncate error messages.

With false, the default.

ts
var x:var x: {
    propertyWithAnExceedinglyLongName1: string;
    propertyWithAnExceedinglyLongName2: string;
    propertyWithAnExceedinglyLongName3: string;
    propertyWithAnExceedinglyLongName4: string;
    propertyWithAnExceedinglyLongName5: string;
} {
  propertyWithAnExceedinglyLongName1:(property) propertyWithAnExceedinglyLongName1: string string;
  propertyWithAnExceedinglyLongName2:(property) propertyWithAnExceedinglyLongName2: string string;
  propertyWithAnExceedinglyLongName3:(property) propertyWithAnExceedinglyLongName3: string string;
  propertyWithAnExceedinglyLongName4:(property) propertyWithAnExceedinglyLongName4: string string;
  propertyWithAnExceedinglyLongName5:(property) propertyWithAnExceedinglyLongName5: string string;
};

// String representation of type of 'x' should be truncated in error message
var s:var s: string string = x;var x: {
    propertyWithAnExceedinglyLongName1: string;
    propertyWithAnExceedinglyLongName2: string;
    propertyWithAnExceedinglyLongName3: string;
    propertyWithAnExceedinglyLongName4: string;
    propertyWithAnExceedinglyLongName5: string;
}
Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.2322
2454Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.

With true

ts
var x:var x: {
    propertyWithAnExceedinglyLongName1: string;
    propertyWithAnExceedinglyLongName2: string;
    propertyWithAnExceedinglyLongName3: string;
    propertyWithAnExceedinglyLongName4: string;
    propertyWithAnExceedinglyLongName5: string;
} {
  propertyWithAnExceedinglyLongName1:(property) propertyWithAnExceedinglyLongName1: string string;
  propertyWithAnExceedinglyLongName2:(property) propertyWithAnExceedinglyLongName2: string string;
  propertyWithAnExceedinglyLongName3:(property) propertyWithAnExceedinglyLongName3: string string;
  propertyWithAnExceedinglyLongName4:(property) propertyWithAnExceedinglyLongName4: string string;
  propertyWithAnExceedinglyLongName5:(property) propertyWithAnExceedinglyLongName5: string string;
};

// String representation of type of 'x' should be truncated in error message
var s:var s: string string = x;var x: {
    propertyWithAnExceedinglyLongName1: string;
    propertyWithAnExceedinglyLongName2: string;
    propertyWithAnExceedinglyLongName3: string;
    propertyWithAnExceedinglyLongName4: string;
    propertyWithAnExceedinglyLongName5: string;
}
Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.2322
2454Type '{ propertyWithAnExceedinglyLongName1: string; propertyWithAnExceedinglyLongName2: string; propertyWithAnExceedinglyLongName3: string; propertyWithAnExceedinglyLongName4: string; propertyWithAnExceedinglyLongName5: string; }' is not assignable to type 'string'.
Variable 'x' is used before being assigned.

Disables the automatic inclusion of any library files. If this option is set, lib is ignored.

By default, TypeScript will examine the initial set of files for import and <reference directives and add these resolved files to your program.

If noResolve isn’t set, this process doesn’t happen. However, import statements are still checked to see if they resolve to a valid module, so you’ll need to make sure this is satisfied by some other means.

Do not emit declarations for code that has an @internal annotation in it’s JSDoc comment. This is an internal compiler option; use at your own risk, because the compiler does not check that the result is valid. If you are searching for a tool to handle additional levels of visibility within your d.ts files, look at api-extractor.

ts
/**
 * Days available in a week
 * @internal
 */
export const daysInAWeek const daysInAWeek: 7= 7;

/** Calculate how much someone earns in a week */
export function weeklySalary(function weeklySalary(dayRate: number): numberdayRate:(parameter) dayRate: number number) {
  return daysInAWeek const daysInAWeek: 7* dayRate;(parameter) dayRate: number
}

With the flag set to false (default):

ts
/**
 * Days available in a week
 * @internal
 */
export declare const daysInAWeek = 7;
/** Calculate how much someone earns in a week */
export declare function weeklySalary(dayRate: number): number;

With stripInternal set to true the d.ts emitted will be redacted.

ts
/** Calculate how much someone earns in a week */
export declare function weeklySalary(dayRate: number): number;

The JavaScript output is still the same.

To avoid a possible memory bloat issue when working with very large JavaScript projects, there is an upper limit to the amount of memory TypeScript will allocate. Turning this flag on will remove the limit.

When working with composite TypeScript projects, this option provides a way to go back to the pre-3.7 behavior where d.ts files were used to as the boundaries between modules. In 3.7 the source of truth is now your TypeScript files.

When working with composite TypeScript projects, this option provides a way to declare that you do not want a project to be included when using features like find all references or jump to definition in an editor.

This flag is something you can use to increase responsiveness in large composite projects.

You shouldn’t need this. By default, when emitting a module file to a non-ES6 target, TypeScript emits a "use strict"; prologue at the top of the file. This setting disables the prologue.

js
define(["require", "exports"], function (require, exports) {
    exports.__esModule = true;
    function fn() { }
    exports.fn = fn;
});

js
define(["require", "exports"], function (require, exports) {
    "use strict";
    exports.__esModule = true;
    function fn() { }
    exports.fn = fn;
});

Instead of importing helpers with importHelpers, you can provide implementations in the global scope for the helpers you use and completely turn off emitting of helper functions.

For example, using this async function in ES5 requires a await-like function and generator-like function to run:

ts
const getAPI const getAPI: (url: string) => Promise<{}>= async (url:(parameter) url: string string) => {
  // Get API
  return {};
};

Which creates quite a lot of JavaScript:

js
"use strict";
var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments || [])).next());
    });
};
var __generator = (this && this.__generator) || function (thisArg, body) {
    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
    function verb(n) { return function (v) { return step([n, v]); }; }
    function step(op) {
        if (f) throw new TypeError("Generator is already executing.");
        while (_) try {
            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
            if (y = 0, t) op = [op[0] & 2, t.value];
            switch (op[0]) {
                case 0: case 1: t = op; break;
                case 4: _.label++; return { value: op[1], done: false };
                case 5: _.label++; y = op[1]; op = [0]; continue;
                case 7: op = _.ops.pop(); _.trys.pop(); continue;
                default:
                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
                    if (t[2]) _.ops.pop();
                    _.trys.pop(); continue;
            }
            op = body.call(thisArg, _);
        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
    }
};
var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {
    return __generator(this, function (_a) {
        // Get API
        return [2 /*return*/, {}];
    });
}); };

Which can be switched out with your own globals via this flag:

js
"use strict";
var getAPI = function (url) { return __awaiter(void 0, void 0, void 0, function () {
    return __generator(this, function (_a) {
        // Get API
        return [2 /*return*/, {}];
    });
}); };

Do not emit compiler output files like JavaScript source code, source-maps or declarations if any errors were reported.

This defaults to false, making it easier to work with TypeScript in a watch-like environment where you may want to see results of changes to your code in another environment before making sure all errors are resolved.

Do not erase const enum declarations in generated code. const enums provide a way to reduce the overall memory footprint of your application at runtime by emitting the enum value instead of a reference.

For example with this TypeScript:

ts
const enum Album const enum Album
  JimmyEatWorldFutures (enum member) Album.JimmyEatWorldFutures = 1= 1,
  TubRingZooHypothesis (enum member) Album.TubRingZooHypothesis = 2= 2,
  DogFashionDiscoAdultery (enum member) Album.DogFashionDiscoAdultery = 3= 3
}

const selectedAlbum const selectedAlbum: Album.JimmyEatWorldFutures= Album.const enum AlbumJimmyEatWorldFutures;(enum member) Album.JimmyEatWorldFutures = 1
if (selectedAlbum const selectedAlbum: Album.JimmyEatWorldFutures=== Album.const enum AlbumJimmyEatWorldFutures)(enum member) Album.JimmyEatWorldFutures = 1 {
  console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): void"That is a great choice.");
}

The default const enum behavior is to convert any Album.Something to the corresponding number literal, and to remove a reference to the enum from the JavaScript completely.

js
"use strict";
const selectedAlbum = 1 /* JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}

With preserveConstEnums set to true, the enum exists at runtime and the numbers are still emitted.

js
"use strict";
var Album;
(function (Album) {
    Album[Album["JimmyEatWorldFutures"] = 1] = "JimmyEatWorldFutures";
    Album[Album["TubRingZooHypothesis"] = 2] = "TubRingZooHypothesis";
    Album[Album["DogFashionDiscoAdultery"] = 3] = "DogFashionDiscoAdultery";
})(Album || (Album = {}));
const selectedAlbum = 1 /* JimmyEatWorldFutures */;
if (selectedAlbum === 1 /* JimmyEatWorldFutures */) {
    console.log("That is a great choice.");
}

This essentially makes such const enums a source-code feature only, with no runtime traces.

Offers a way to configure the root directory for where declaration files are emitted.

example
├── index.ts
├── package.json
└── tsconfig.json

with this tsconfig.json:

json
{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./types"
  }
}

Would place the d.ts for the index.ts in a types folder:

example
├── index.js
├── index.ts
├── package.json
├── tsconfig.json
└── types
    └── index.d.ts

Skip type checking of declaration files.

This can save time during compilation at the expense of type-system accuracy. For example, two libraries could define two copies of the same type in an inconsistent way. Rather than doing a full check of all d.ts files, TypeScript will type check the code you specifically refer to in your app’s source code.

A common case where you might think to use skipLibCheck is when there are two copies of a library’s types in your node_modules. In these cases, you should consider using a feature like yarn’s resolutions to ensure there is only one copy of that dependency in your tree or investigate how to ensure there is only one copy by understanding the dependency resolution to fix the issue without additional tooling.

Set to false to disable warnings about unused labels.

Labels are very rare in JavaScript and typically indicate an attempt to write an object literal:

ts
function verifyAge(function verifyAge(age: number): voidage:(parameter) age: number number) {
  // Forgot 'return' statement
  if (age (parameter) age: number> 18) {
    verified:true;
Unused label.7028Unused label.  }
}

Set to false to disable warnings about unreachable code. These warnings are only about code which is provably unreachable due to the use of JavaScript syntax, for example:

ts
function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

With "allowUnreachableCode": false:

ts
function fn(function fn(n: number): booleann:(parameter) n: number number) {
  if (n (parameter) n: number> 5) {
    return true;
  } else {
    return false;
  }
  return true;
Unreachable code detected.7027Unreachable code detected.}

This does not affect errors on the basis of code which appears to be unreachable due to type analysis.

This disables reporting of excess property errors, such as the one shown in the following example:

ts
type Point type Point = {
    x: number;
    y: number;
}= { x:(property) x: number number; y:(property) y: number number };
const p:const p: Point Point type Point = {
    x: number;
    y: number;
}= { x:(property) x: number1, y:(property) y: number3, m:(property) m: number10 };
Type '{ x: number; y: number; m: number; }' is not assignable to type 'Point'.
  Object literal may only specify known properties, and 'm' does not exist in type 'Point'.2322Type '{ x: number; y: number; m: number; }' is not assignable to type 'Point'.
  Object literal may only specify known properties, and 'm' does not exist in type 'Point'.

This flag was added to help people migrate to the stricter checking of new object literals in TypeScript 1.6.

We don’t recommend using this flag in a modern codebase, you can suppress one-off cases where you need it using // @ts-ignore.

Turning noImplicitAny on suppresses reporting the error about implicit anys when indexing into objects, as shown in the following example:

ts
const obj const obj: {
    x: number;
}= { x:(property) x: number10 };
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidobj[const obj: {
    x: number;
}"foo"]);
Element implicitly has an 'any' type because expression of type '"foo"' can't be used to index type '{ x: number; }'.
  Property 'foo' does not exist on type '{ x: number; }'.7053Element implicitly has an 'any' type because expression of type '"foo"' can't be used to index type '{ x: number; }'.
  Property 'foo' does not exist on type '{ x: number; }'.

Using suppressImplicitAnyIndexErrors is quite a drastic approach. It is recommended to use a @ts-ignore comment instead:

ts
const obj const obj: {
    x: number;
}= { x:(property) x: number10 };
// @ts-ignore
console.var console: Consolelog((method) Console.log(message?: any, ...optionalParams: any[]): voidobj[const obj: {
    x: number;
}"foo"]);

TypeScript follows the case sensitivity rules of the file system it’s running on. This can be problematic if some developers are working in a case-sensitive file system and others aren’t. If a file attempts to import fileManager.ts by specifying ./FileManager.ts the file will be found in a case-insensitive file system, but not on a case-sensitive file system.

When this option is set, TypeScript will issue an error if a program tries to include a file by a casing different from the casing on disk.

The maximum dependency depth to search under node_modules and load JavaScript files.

This flag is can only be used when allowJs is enabled, and is used if you want to have TypeScript infer types for all of the JavaScript inside your node_modules.

Ideally this should stay at 0 (the default), and d.ts files should be used to explicitly define the shape of modules. However, there are cases where you may want to turn this on at the expense of speed and potential accuracy.

TypeScript will unify type parameters when comparing two generic functions functions

ts
type A type A = <T, U>(x: T, y: U) => [T, U]= <T,(type parameter) T in <T, U>(x: T, y: U): [T, U] U>(type parameter) U in <T, U>(x: T, y: U): [T, U](x:(parameter) x: T T,(type parameter) T in <T, U>(x: T, y: U): [T, U] y:(parameter) y: U U)(type parameter) U in <T, U>(x: T, y: U): [T, U]=> [T,(type parameter) T in <T, U>(x: T, y: U): [T, U] U](type parameter) U in <T, U>(x: T, y: U): [T, U]
type B type B = <S>(x: S, y: S) => [S, S]= <S>(type parameter) S in <S>(x: S, y: S): [S, S](x:(parameter) x: S S,(type parameter) S in <S>(x: S, y: S): [S, S] y:(parameter) y: S S)(type parameter) S in <S>(x: S, y: S): [S, S]=> [S,(type parameter) S in <S>(x: S, y: S): [S, S] S](type parameter) S in <S>(x: S, y: S): [S, S]

function f(function f(a: A, b: B): voida:(parameter) a: A A,type A = <T, U>(x: T, y: U) => [T, U] b:(parameter) b: B B)type B = <S>(x: S, y: S) => [S, S] {
  b (parameter) b: B= a;(parameter) a: A// Ok
  a (parameter) a: A= b;(parameter) b: B// Error
Type 'B' is not assignable to type 'A'.
  Types of parameters 'y' and 'y' are incompatible.
    Type 'U' is not assignable to type 'T'.
      'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.2322Type 'B' is not assignable to type 'A'.
  Types of parameters 'y' and 'y' are incompatible.
    Type 'U' is not assignable to type 'T'.
      'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.}

This flag can be used to remove that check.

This flag is used as part of migrating to the upcoming standard version of class fields. TypeScript introduced class fields many years before it was ratified in TC39. The latest version of the upcoming specification has a different runtime behavior to TypeScript’s implementation but the same syntax.

This flag switches to the upcoming ECMA runtime behavior.

You can read more about the transition in the 3.7 release notes.

This flag changes the keyof type operator to return string instead of string | number when applied to a type with a string index signature.

This flag is used to help people keep this behavior from before TypeScript 2.9’s release.

Whether to keep outdated console output in watch mode instead of clearing the screen every time a change happened.

Stylize errors and messages using color and context, this is on by default — offers you a chance to have less terse, single colored messages from the compiler.