TypeScript 中文文档：Documentation - Modules

Module syntax

The TypeScript compiler recognizes standard ECMAScript module syntax in TypeScript and JavaScript files and many forms of CommonJS syntax in JavaScript files.

There are also a few TypeScript-specific syntax extensions that can be used in TypeScript files and/or JSDoc comments.

Importing and exporting TypeScript-specific declarations

Type aliases, interfaces, enums, and namespaces can be exported from a module with an export modifier, like any standard JavaScript declaration:

ts
// Standard JavaScript syntax...
export function f() {}
// ...extended to type declarations
export type SomeType = /* ... */;
export interface SomeInterface { /* ... */ }

They can also be referenced in named exports, even alongside references to standard JavaScript declarations:

ts
export { f, SomeType, SomeInterface };

Exported types (and other TypeScript-specific declarations) can be imported with standard ECMAScript imports:

ts
import { f, SomeType, SomeInterface } from "./module.js";

When using namespace imports or exports, exported types are available on the namespace when referenced in a type position:

ts
import * as mod from "./module.js";
mod.f();
mod.SomeType; // Property 'SomeType' does not exist on type 'typeof import("./module.js")'
let x: mod.SomeType; // Ok

Type-only imports and exports

When emitting imports and exports to JavaScript, by default, TypeScript automatically elides (does not emit) imports that are only used in type positions and exports that only refer to types. Type-only imports and exports can be used to force this behavior and make the elision explicit. Import declarations written with import type, export declarations written with export type { ... }, and import or export specifiers prefixed with the type keyword are all guaranteed to be elided from the output JavaScript.

ts
// @Filename: main.ts
import { f, type SomeInterface } from "./module.js";
import type { SomeType } from "./module.js";
class C implements SomeInterface {
  constructor(p: SomeType) {
    f();
  }
}
export type { C };
// @Filename: main.js
import { f } from "./module.js";
class C {
  constructor(p) {
    f();
  }
}

Even values can be imported with import type, but since they won’t exist in the output JavaScript, they can only be used in non-emitting positions:

ts
import type { f } from "./module.js";
f(); // 'f' cannot be used as a value because it was imported using 'import type'
let otherFunction: typeof f = () => {}; // Ok

A type-only import declaration may not declare both a default import and named bindings, since it appears ambiguous whether type applies to the default import or to the entire import declaration. Instead, split the import declaration into two, or use default as a named binding:

ts
import type fs, { BigIntOptions } from "fs";
//          ^^^^^^^^^^^^^^^^^^^^^
// Error: A type-only import can specify a default import or named bindings, but not both.
import type { default as fs, BigIntOptions } from "fs"; // Ok

`import()` types

TypeScript provides a type syntax similar to JavaScript’s dynamic import for referencing the type of a module without writing an import declaration:

ts
// Access an exported type:
type WriteFileOptions = import("fs").WriteFileOptions;
// Access the type of an exported value:
type WriteFileFunction = typeof import("fs").writeFile;

This is especially useful in JSDoc comments in JavaScript files, where it’s not possible to import types otherwise:

ts
/** @type {import("webpack").Configuration} */
module.exports = {
  // ...
}

`export =` and `import = require()`

When emitting CommonJS modules, TypeScript files can use a direct analog of module.exports = ... and const mod = require("...") JavaScript syntax:

ts
// @Filename: main.ts
import fs = require("fs");
export = fs.readFileSync("...");
// @Filename: main.js
"use strict";
const fs = require("fs");
module.exports = fs.readFileSync("...");

This syntax was used over its JavaScript counterparts since variable declarations and property assignments could not refer to TypeScript types, whereas special TypeScript syntax could:

ts
// @Filename: a.ts
interface Options { /* ... */ }
module.exports = Options; // Error: 'Options' only refers to a type, but is being used as a value here.
export = Options; // Ok
// @Filename: b.ts
const Options = require("./a");
const options: Options = { /* ... */ }; // Error: 'Options' refers to a value, but is being used as a type here.
// @Filename: c.ts
import Options = require("./a");
const options: Options = { /* ... */ }; // Ok

Ambient modules

TypeScript supports a syntax in script (non-module) files for declaring a module that exists in the runtime but has no corresponding file. These ambient modules usually represent runtime-provided modules, like "fs" or "path" in Node.js:

ts
declare module "path" {
  export function normalize(p: string): string;
  export function join(...paths: any[]): string;
  export var sep: string;
}

Once an ambient module is loaded into a TypeScript program, TypeScript will recognize imports of the declared module in other files:

ts
// 👇 Ensure the ambient module is loaded -
//    may be unnecessary if path.d.ts is included
//    by the project tsconfig.json somehow.
/// <reference path="path.d.ts" />
import { normalize, join } from "path";

Ambient module declarations are easy to confuse with module augmentations since they use identical syntax. This module declaration syntax becomes a module augmentation when the file is a module, meaning it has a top-level import or export statement (or is affected by --moduleDetection force or auto):

ts
// Not an ambient module declaration anymore!
export {};
declare module "path" {
  export function normalize(p: string): string;
  export function join(...paths: any[]): string;
  export var sep: string;
}

Ambient modules may use imports inside the module declaration body to refer to other modules without turning the containing file into a module (which would make the ambient module declaration a module augmentation):

ts
declare module "m" {
  // Moving this outside "m" would totally change the meaning of the file!
  import { SomeType } from "other";
  export function f(): SomeType;
}

A pattern ambient module contains a single * wildcard character in its name, matching zero or more characters in import paths. This can be useful for declaring modules provided by custom loaders:

ts
declare module "*.html" {
  const content: string;
  export default content;
}

The `module` compiler option

This section discusses the details of each module compiler option value. See the Module output format theory section for more background on what the option is and how it fits into the overall compilation process. In brief, the module compiler option was historically only used to control the output module format of emitted JavaScript files. The more recent node16 and nodenext values, however, describe a wide range of characteristics of Node.js’s module system, including what module formats are supported, how the module format of each file is determined, and how different module formats interoperate.

`node16`, `nodenext`

Node.js supports both CommonJS and ECMAScript modules, with specific rules for which format each file can be and how the two formats are allowed to interoperate. node16 and nodenext describe the full range of behavior for Node.js’s dual-format module system, and emit files in either CommonJS or ESM format. This is different from every other module option, which are runtime-agnostic and force all output files into a single format, leaving it to the user to ensure the output is valid for their runtime.

A common misconception is that node16 and nodenext only emit ES modules. In reality, node16 and nodenext describe versions of Node.js that support ES modules, not just projects that use ES modules. Both ESM and CommonJS emit are supported, based on the detected module format of each file. Because node16 and nodenext are the only module options that reflect the complexities of Node.js’s dual module system, they are the only correct module options for all apps and libraries that are intended to run in Node.js v12 or later, whether they use ES modules or not.

node16 and nodenext are currently identical, with the exception that they imply different target option values. If Node.js makes significant changes to its module system in the future, node16 will be frozen while nodenext will be updated to reflect the new behavior.

Module format detection

.mts/.mjs/.d.mts files are always ES modules.
.cts/.cjs/.d.cts files are always CommonJS modules.
.ts/.tsx/.js/.jsx/.d.ts files are ES modules if the nearest ancestor package.json file contains "type": "module", otherwise CommonJS modules.

The detected module format of input .ts/.tsx/.mts/.cts files determines the module format of the emitted JavaScript files. So, for example, a project consisting entirely of .ts files will emit all CommonJS modules by default under --module nodenext, and can be made to emit all ES modules by adding "type": "module" to the project package.json.

Interoperability rules

When an ES module references a CommonJS module:
- The module.exports of the CommonJS module is available as a default import to the ES module.
- Properties (other than default) of the CommonJS module’s module.exports may or may not be available as named imports to the ES module. Node.js attempts to make them available via static analysis. TypeScript cannot know from a declaration file whether that static analysis will succeed, and optimistically assumes it will. This limits TypeScript’s ability to catch named imports that may crash at runtime. See #54018 for more details.
When a CommonJS module references an ES module:
- require cannot reference an ES module. For TypeScript, this includes import statements in files that are detected to be CommonJS modules, since those import statements will be transformed to require calls in the emitted JavaScript.
- A dynamic import() call may be used to import an ES module. It returns a Promise of the module’s Module Namespace Object (what you’d get from import * as ns from "./module.js" from another ES module).

Emit

The emit format of each file is determined by the detected module format of each file. ESM emit is similar to --module esnext, but has a special transformation for import x = require("..."), which is not allowed in --module esnext:

ts
// @Filename: main.ts
import x = require("mod");

js
// @Filename: main.js
import { createRequire as _createRequire } from "module";
const __require = _createRequire(import.meta.url);
const x = __require("mod");

CommonJS emit is similar to --module commonjs, but dynamic import() calls are not transformed. Emit here is shown with esModuleInterop enabled:

ts
// @Filename: main.ts
import fs from "fs"; // transformed
const dynamic = import("mod"); // not transformed

js
// @Filename: main.js
"use strict";
var __importDefault = (this && this.__importDefault) || function (mod) {
    return (mod && mod.__esModule) ? mod : { "default": mod };
};
Object.defineProperty(exports, "__esModule", { value: true });
const fs_1 = __importDefault(require("fs")); // transformed
const dynamic = import("mod"); // not transformed

Implied and enforced options

--module nodenext or node16 implies and enforces the moduleResolution with the same name.
--module nodenext implies --target esnext.
--module node16 implies --target es2022.
--module nodenext or node16 implies --esModuleInterop.

Summary

node16 and nodenext are the only correct module options for all apps and libraries that are intended to run in Node.js v12 or later, whether they use ES modules or not.
node16 and nodenext emit files in either CommonJS or ESM format, based on the detected module format of each file.
Node.js’s interoperability rules between ESM and CJS are reflected in type checking.
ESM emit transforms import x = require("...") to a require call constructed from a createRequire import.
CommonJS emit leaves dynamic import() calls untransformed, so CommonJS modules can asynchronously import ES modules.

`preserve`

In --module preserve (added in TypeScript 5.4), ECMAScript imports and exports written in input files are preserved in the output, and CommonJS-style import x = require("...") and export = ... statements are emitted as CommonJS require and module.exports. In other words, the format of each individual import or export statement is preserved, rather than being coerced into a single format for the whole compilation (or even a whole file).

While it’s rare to need to mix imports and require calls in the same file, this module mode best reflects the capabilities of most modern bundlers, as well as the Bun runtime.

Why care about TypeScript’s module emit with a bundler or with Bun, where you’re likely also setting noEmit? TypeScript’s type checking and module resolution behavior are affected by the module format that it would emit. Setting module gives TypeScript information about how your bundler or runtime will process imports and exports, which ensures that the types you see on imported values accurately reflect what will happen at runtime or after bundling. See --moduleResolution bundler for more discussion.

Examples

ts
import x, { y, z } from "mod";
import mod = require("mod");
const dynamic = import("mod");
export const e1 = 0;
export default "default export";

js
import x, { y, z } from "mod";
const mod = require("mod");
const dynamic = import("mod");
export const e1 = 0;
export default "default export";

Implied and enforced options

--module preserve implies --moduleResolution bundler.
--module preserve implies --esModuleInterop.

The option --esModuleInterop is enabled by default in --module preserve only for its type checking behavior. Since imports never transform into require calls in --module preserve, --esModuleInterop does not affect the emitted JavaScript.

`es2015`, `es2020`, `es2022`, `esnext`

Summary

Use esnext with --moduleResolution bundler for bundlers, Bun, and tsx.
Do not use for Node.js. Use node16 or nodenext with "type": "module" in package.json to emit ES modules for Node.js.
import mod = require("mod") is not allowed in non-declaration files.
es2020 adds support for import.meta properties.
es2022 adds support for top-level await.
esnext is a moving target that may include support for Stage 3 proposals to ECMAScript modules.
Emitted files are ES modules, but dependencies may be any format.

Examples

ts
// @Filename: main.ts
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

js
// @Filename: main.js
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

`commonjs`

Summary

You probably shouldn’t use this. Use node16 or nodenext to emit CommonJS modules for Node.js.
Emitted files are CommonJS modules, but dependencies may be any format.
Dynamic import() is transformed to a Promise of a require() call.
esModuleInterop affects the output code for default and namespace imports.

Examples

Output is shown with esModuleInterop: false.

ts
// @Filename: main.ts
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

js
// @Filename: main.js
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.e1 = void 0;
const mod_1 = require("mod");
const mod = require("mod");
const dynamic = Promise.resolve().then(() => require("mod"));
console.log(mod_1.default, mod_1.y, mod_1.z, mod);
exports.e1 = 0;
exports.default = "default export";

ts
// @Filename: main.ts
import mod = require("mod");
console.log(mod);
export = {
    p1: true,
    p2: false
};

js
// @Filename: main.js
"use strict";
const mod = require("mod");
console.log(mod);
module.exports = {
    p1: true,
    p2: false
};

`system`

Summary

Designed for use with the SystemJS module loader.

Examples

ts
// @Filename: main.ts
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

js
// @Filename: main.js
System.register(["mod"], function (exports_1, context_1) {
    "use strict";
    var mod_1, mod, dynamic, e1;
    var __moduleName = context_1 && context_1.id;
    return {
        setters: [
            function (mod_1_1) {
                mod_1 = mod_1_1;
                mod = mod_1_1;
            }
        ],
        execute: function () {
            dynamic = context_1.import("mod");
            console.log(mod_1.default, mod_1.y, mod_1.z, mod, dynamic);
            exports_1("e1", e1 = 0);
            exports_1("default", "default export");
        }
    };
});

`amd`

Summary

Designed for AMD loaders like RequireJS.
You probably shouldn’t use this. Use a bundler instead.
Emitted files are AMD modules, but dependencies may be any format.
Supports outFile.

Examples

ts
// @Filename: main.ts
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

js
// @Filename: main.js
define(["require", "exports", "mod", "mod"], function (require, exports, mod_1, mod) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.e1 = void 0;
    const dynamic = new Promise((resolve_1, reject_1) => { require(["mod"], resolve_1, reject_1); });
    console.log(mod_1.default, mod_1.y, mod_1.z, mod, dynamic);
    exports.e1 = 0;
    exports.default = "default export";
});

`umd`

Summary

Designed for AMD or CommonJS loaders.
Does not expose a global variable like most other UMD wrappers.
You probably shouldn’t use this. Use a bundler instead.
Emitted files are UMD modules, but dependencies may be any format.

Examples

ts
// @Filename: main.ts
import x, { y, z } from "mod";
import * as mod from "mod";
const dynamic = import("mod");
console.log(x, y, z, mod, dynamic);
export const e1 = 0;
export default "default export";

js
// @Filename: main.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", "mod", "mod"], factory);
    }
})(function (require, exports) {
    "use strict";
    var __syncRequire = typeof module === "object" && typeof module.exports === "object";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.e1 = void 0;
    const mod_1 = require("mod");
    const mod = require("mod");
    const dynamic = __syncRequire ? Promise.resolve().then(() => require("mod")) : new Promise((resolve_1, reject_1) => { require(["mod"], resolve_1, reject_1); });
    console.log(mod_1.default, mod_1.y, mod_1.z, mod, dynamic);
    exports.e1 = 0;
    exports.default = "default export";
});

The `moduleResolution` compiler option

This section describes module resolution features and processes shared by multiple moduleResolution modes, then specifies the details of each mode. See the Module resolution theory section for more background on what the option is and how it fits into the overall compilation process. In brief, moduleResolution controls how TypeScript resolves module specifiers (string literals in import/export/require statements) to files on disk, and should be set to match the module resolver used by the target runtime or bundler.

Common features and processes

File extension substitution

TypeScript always wants to resolve internally to a file that can provide type information, while ensuring that the runtime or bundler can use the same path to resolve to a file that provides a JavaScript implementation. For any module specifier that would, according to the moduleResolution algorithm specified, trigger a lookup of a JavaScript file in the runtime or bundler, TypeScript will first try to find a TypeScript implementation file or type declaration file with the same name and analagous file extension.

Runtime lookup	TypeScript lookup #1	TypeScript lookup #2	TypeScript lookup #3	TypeScript lookup #4	TypeScript lookup #5
`/mod.js`	`/mod.ts`	`/mod.tsx`	`/mod.d.ts`	`/mod.js`	`./mod.jsx`
`/mod.mjs`	`/mod.mts`	`/mod.d.mts`	`/mod.mjs`
`/mod.cjs`	`/mod.cts`	`/mod.d.cts`	`/mod.cjs`

Note that this behavior is independent of the actual module specifier written in the import. This means that TypeScript can resolve to a .ts or .d.ts file even if the module specifier explicitly uses a .js file extension:

ts
import x from "./mod.js";
// Runtime lookup: "./mod.js"
// TypeScript lookup #1: "./mod.ts"
// TypeScript lookup #2: "./mod.d.ts"
// TypeScript lookup #3: "./mod.js"

See TypeScript imitates the host’s module resolution, but with types for an explanation of why TypeScript’s module resolution works this way.

Relative file path resolution

All of TypeScript’s moduleResolution algorithms support referencing a module by a relative path that includes a file extension (which will be substituted according to the rules above):

ts
// @Filename: a.ts
export {};
// @Filename: b.ts
import {} from "./a.js"; // ✅ Works in every `moduleResolution`

Extensionless relative paths

In some cases, the runtime or bundler allows omitting a .js file extension from a relative path. TypeScript supports this behavior where the moduleResolution setting and the context indicate that the runtime or bundler supports it:

ts
// @Filename: a.ts
export {};
// @Filename: b.ts
import {} from "./a";

If TypeScript determines that the runtime will perform a lookup for ./a.js given the module specifier "./a", then ./a.js will undergo extension substitution, and resolve to the file a.ts in this example.

Extensionless relative paths are not supported in import paths in Node.js, and are not always supported in file paths specified in package.json files. TypeScript currently never supports omitting a .mjs/.mts or .cjs/.cts file extension, even though some runtimes and bundlers do.

Directory modules (index file resolution)

In some cases, a directory, rather than a file, can be referenced as a module. In the simplest and most common case, this involves the runtime or bundler looking for an index.js file in a directory. TypeScript supports this behavior where the moduleResolution setting and the context indicate that the runtime or bundler supports it:

ts
// @Filename: dir/index.ts
export {};
// @Filename: b.ts
import {} from "./dir";

If TypeScript determines that the runtime will perform a lookup for ./dir/index.js given the module specifier "./dir", then ./dir/index.js will undergo extension substitution, and resolve to the file dir/index.ts in this example.

Directory modules may also contain a package.json file, where resolution of the "main" and "types" fields are supported, and take precedence over index.js lookups. The "typesVersions" field is also supported in directory modules.

Note that directory modules are not the same as node_modules packages and only support a subset of the features available to packages, and are not supported at all in some contexts. Node.js considers them a legacy feature.

`paths`

Overview

TypeScript offers a way to override the compiler’s module resolution for bare specifiers with the paths compiler option. While the feature was originally designed to be used with the AMD module loader (a means of running modules in the browser before ESM existed or bundlers were widely used), it still has uses today when a runtime or bundler supports module resolution features that TypeScript does not model. For example, when running Node.js with --experimental-network-imports, you can manually specify a local type definition file for a specific https:// import:

json
{
  "compilerOptions": {
    "module": "nodenext",
    "paths": {
      "https://esm.sh/lodash@4.17.21": ["./node_modules/@types/lodash/index.d.ts"]
    }
  }
}

ts
// Typed by ./node_modules/@types/lodash/index.d.ts due to `paths` entry
import { add } from "https://esm.sh/lodash@4.17.21";

It’s also common for apps built with bundlers to define convenience path aliases in their bundler configuration, and then inform TypeScript of those aliases with paths:

json
{
  "compilerOptions": {
    "module": "esnext",
    "moduleResolution": "bundler",
    "paths": {
      "@app/*": ["./src/*"]
    }
  }
}

`paths` does not affect emit

The paths option does not change the import path in the code emitted by TypeScript. Consequently, it’s very easy to create path aliases that appear to work in TypeScript but will crash at runtime:

json
{
  "compilerOptions": {
    "module": "nodenext",
    "paths": {
      "node-has-no-idea-what-this-is": ["./oops.ts"]
    }
  }
}

ts
// TypeScript: ✅
// Node.js: 💥
import {} from "node-has-no-idea-what-this-is";

While it’s ok for bundled apps to set up paths, it’s very important that published libraries do not, since the emitted JavaScript will not work for consumers of the library without those users setting up the same aliases for both TypeScript and their bundler. Both libraries and apps can consider package.json "imports" as a standard replacement for convenience paths aliases.

`paths` should not point to monorepo packages or node_modules packages

While module specifiers that match paths aliases are bare specifiers, once the alias is resolved, module resolution proceeds on the resolved path as a relative path. Consequently, resolution features that happen for node_modules package lookups, including package.json "exports" field support, do not take effect when a paths alias is matched. This can lead to surprising behavior if paths is used to point to a node_modules package:

ts
{
  "compilerOptions": {
    "paths": {
      "pkg": ["./node_modules/pkg/dist/index.d.ts"],
      "pkg/*": ["./node_modules/pkg/*"]
    }
  }
}

While this configuration may simulate some of the behavior of package resolution, it overrides any main, types, exports, and typesVersions the package’s package.json file defines, and imports from the package may fail at runtime.

The same caveat applies to packages referencing each other in a monorepo. Instead of using paths to make TypeScript artificially resolve "@my-scope/lib" to a sibling package, it’s best to use workspaces via npm, yarn, or pnpm to symlink your packages into node_modules, so both TypeScript and the runtime or bundler perform real node_modules package lookups. This is especially important if the monorepo packages will be published to npm—the packages will reference each other via node_modules package lookups once installed by users, and using workspaces allows you to test that behavior during local development.

Relationship to `baseUrl`

When baseUrl is provided, the values in each paths array are resolved relative to the baseUrl. Otherwise, they are resolved relative to the tsconfig.json file that defines them.

Wildcard substitutions

paths patterns can contain a single * wildcard, which matches any string. The * token can then be used in the file path values to substitute the matched string:

json
{
  "compilerOptions": {
    "paths": {
      "@app/*": ["./src/*"]
    }
  }
}

When resolving an import of "@app/components/Button", TypeScript will match on @app/*, binding * to components/Button, and then attempt to resolve the path ./src/components/Button relative to the tsconfig.json path. The remainder of this lookup will follow the same rules as any other relative path lookup according to the moduleResolution setting.

When multiple patterns match a module specifier, the pattern with the longest matching prefix before any * token is used:

json
{
  "compilerOptions": {
    "paths": {
      "*": ["./src/foo/one.ts"],
      "foo/*": ["./src/foo/two.ts"],
      "foo/bar": ["./src/foo/three.ts"]
    }
  }
}

When resolving an import of "foo/bar", all three paths patterns match, but the last is used because "foo/bar" is longer than "foo/" and "".

Fallbacks

Multiple file paths can be provided for a path mapping. If resolution fails for one path, the next one in the array will be attempted until resolution succeeds or the end of the array is reached.

json
{
  "compilerOptions": {
    "paths": {
      "*": ["./vendor/*", "./types/*"]
    }
  }
}

`baseUrl`

baseUrl was designed for use with AMD module loaders. If you aren’t using an AMD module loader, you probably shouldn’t use baseUrl. Since TypeScript 4.1, baseUrl is no longer required to use paths and should not be used just to set the directory paths values are resolved from.

The baseUrl compiler option can be combined with any moduleResolution mode and specifies a directory that bare specifiers (module specifiers that don’t begin with ./, ../, or /) are resolved from. baseUrl has a higher precedence than node_modules package lookups in moduleResolution modes that support them.

When performing a baseUrl lookup, resolution proceeds with the same rules as other relative path resolutions. For example, in a moduleResolution mode that supports extensionless relative paths a module specifier "some-file" may resolve to /src/some-file.ts if baseUrl is set to /src.

Resolution of relative module specifiers are never affected by the baseUrl option.

`node_modules` package lookups

Node.js treats module specifiers that aren’t relative paths, absolute paths, or URLs as references to packages that it looks up in node_modules subdirectories. Bundlers conveniently adopted this behavior to allow their users to use the same dependency management system, and often even the same dependencies, as they would in Node.js. All of TypeScript’s moduleResolution options except classic support node_modules lookups. (classic supports lookups in node_modules/@types when other means of resolution fail, but never looks for packages in node_modules directly.) Every node_modules package lookup has the following structure (beginning after higher precedence bare specifier rules, like paths, baseUrl, self-name imports, and package.json "imports" lookups have been exhausted):

For each ancestor directory of the importing file, if a node_modules directory exists within it:
1. If a directory with the same name as the package exists within node_modules:
  1. Attempt to resolve types from the package directory.
  2. If a result is found, return it and stop the search.
2. If a directory with the same name as the package exists within node_modules/@types:
  1. Attempt to resolve types from the @types package directory.
  2. If a result is found, return it and stop the search.
Repeat the previous search through all node_modules directories, but this time, allow JavaScript files as a result, and do not search in @types directories.

All moduleResolution modes (except classic) follow this pattern, while the details of how they resolve from a package directory, once located, differ, and are explained in the following sections.

package.json `"exports"`

When moduleResolution is set to node16, nodenext, or bundler, and resolvePackageJsonExports is not disabled, TypeScript follows Node.js’s package.json "exports" spec when resolving from a package directory triggered by a bare specifier node_modules package lookup.

TypeScript’s implementation for resolving a module specifier through "exports" to a file path follows Node.js exactly. Once a file path is resolved, however, TypeScript will still try multiple file extensions in order to prioritize finding types.

When resolving through conditional "exports", TypeScript always matches the "types" and "default" conditions if present. Additionally, TypeScript will match a versioned types condition in the form "types@{selector}" (where {selector} is a "typesVersions"-compatible version selector) according to the same version-matching rules implemented in "typesVersions". Other non-configurable conditions are dependent on the moduleResolution mode and specified in the following sections. Additional conditions can be configured to match with the customConditions compiler option.

Note that the presence of "exports" prevents any subpaths not explicitly listed or matched by a pattern in "exports" from being resolved.

Example: subpaths, conditions, and extension substitution

Scenario: "pkg/subpath" is requested with conditions ["types", "node", "require"] (determined by moduleResolution setting and the context that triggered the module resolution request) in a package directory with the following package.json:

json
{
  "name": "pkg",
  "exports": {
    ".": {
      "import": "./index.mjs",
      "require": "./index.cjs"
    },
    "./subpath": {
      "import": "./subpath/index.mjs",
      "require": "./subpath/index.cjs"
    }
  }
}