JavaScript has a long history of different ways to handle modularizing code. TypeScript having been around since 2012, has implemented support for a lot of these formats, but over time the community and the JavaScript specification has converged on a format called ES Modules (or ES6 modules). You might know it as the import/export syntax.

ES Modules was added to the JavaScript spec in 2015, and by 2020 had broad support in most web browsers and JavaScript runtimes.

For focus, the handbook will cover both ES Modules and its popular pre-cursor CommonJS module.exports = syntax, and you can find information about the other module patterns in the reference section under Modules.

How JavaScript Modules are Defined

In TypeScript, just as in ECMAScript 2015, any file containing a top-level import or export is considered a module.

Conversely, a file without any top-level import or export declarations is treated as a script whose contents are available in the global scope (and therefore to modules as well).

Modules are executed within their own scope, not in the global scope. This means that variables, functions, classes, etc. declared in a module are not visible outside the module unless they are explicitly exported using one of the export forms. Conversely, to consume a variable, function, class, interface, etc. exported from a different module, it has to be imported using one of the import forms.

Non-modules

Before we start, it’s important to understand what TypeScript considers a module. The JavaScript specification declares that any JavaScript files without an export or top-level await should be considered a script and not a module.

Inside a script file variables and types are declared to be in the shared global scope, and it’s assumed that you’ll either use the --outFile compiler option to join multiple input files into one output file, or use multiple <script> tags in your HTML to load these files (in the correct order!).

If you have a file that doesn’t currently have any imports or exports, but you want to be treated as a module, add the line:

export {};

which will change the file be a module exporting nothing. This syntax works regardless of your module target.

Modules in TypeScript

Additional Reading:
Impatient JS (Modules)
MDN: JavaScript Modules

There are three main things to consider when writing module-based code in TypeScript:

• Syntax: What syntax do I want to use to import and export things?
• Module Resolution: What is the relationship between module names (or paths) and files on disk?
• Module Output Target: What should my emitted JavaScript module look like?

ES Module Syntax

A file can declare a main export via export default:

// @filename: hello.ts
export default function helloWorld() {
  console.log("Hello, world!");
}

This is then imported via:

import hello from "./hello.js";
hello();

In addition to the default export, you can have more than one export of variables and functions via the export by omitting default:

// @filename: maths.ts
export var pi = 3.14;
export let squareTwo = 1.41;
export const phi = 1.61;
export class RandomNumberGenerator {}
export function absolute(num: number) {
  if (num < 0) return num * -1;
  return num;
}

These can be used in another file via the import syntax:

import { pi, phi, absolute } from "./maths.js";
console.log(pi);
const absPhi = absolute(phi);
        
const absPhi: number

Additional Import Syntax

An import can be renamed using a format like import {old as new}:

import { pi as π } from "./maths.js";
console.log(π);
           
(alias) var π: number
import π

You can mix and match the above syntax into a single import:

// @filename: maths.ts
export const pi = 3.14;
export default class RandomNumberGenerator {}
// @filename: app.ts
import RNGen, { pi as π } from "./maths.js";
RNGen;
 
(alias) class RNGen
import RNGen
console.log(π);
           
(alias) const π: 3.14
import π

You can take all of the exported objects and put them into a single namespace using * as name:

// @filename: app.ts
import * as math from "./maths.js";
console.log(math.pi);
const positivePhi = math.absolute(math.phi);
          
const positivePhi: number

You can import a file and not include any variables into your current module via import "./file":

// @filename: app.ts
import "./maths.js";
console.log("3.14");

In this case, the import does nothing. However, all of the code in maths.ts was evaluated, which could trigger side-effects which affect other objects.

TypeScript Specific ES Module Syntax

Types can be exported and imported using the same syntax as JavaScript values:

// @filename: animal.ts
export type Cat = { breed: string; yearOfBirth: number };
export interface Dog {
  breeds: string[];
  yearOfBirth: number;
}
// @filename: app.ts
import { Cat, Dog } from "./animal.js";
type Animals = Cat | Dog;

TypeScript has extended the import syntax with import type which is an import which can only import types.

// @filename: animal.ts
export type Cat = { breed: string; yearOfBirth: number };
'createCatName' cannot be used as a value because it was imported using 'import type'.1361'createCatName' cannot be used as a value because it was imported using 'import type'.
export type Dog = { breeds: string[]; yearOfBirth: number };
export const createCatName = () => "fluffy";
// @filename: valid.ts
import type { Cat, Dog } from "./animal.js";
export type Animals = Cat | Dog;
// @filename: app.ts
import type { createCatName } from "./animal.js";
const name = createCatName();

This syntax allows a non-TypeScript transpiler like Babel, swc or esbuild to know what imports can be safely removed.

ES Module Syntax with CommonJS Behavior

TypeScript has ES Module syntax which directly correlates to a CommonJS and AMD require. Imports using ES Module are for most cases the same as the require from those environments, but this syntax ensures you have a 1 to 1 match in your TypeScript file with the CommonJS output:

import fs = require("fs");
const code = fs.readFileSync("hello.ts", "utf8");

You can learn more about this syntax in the modules reference page.

CommonJS Syntax

CommonJS is the format which most modules on npm are delivered in. Even if you are writing using the ES Modules syntax above, having a brief understanding of how CommonJS syntax works will help you debug easier.

Exporting

Identifiers are exported via setting the exports property on a global called module.

function absolute(num: number) {
  if (num < 0) return num * -1;
  return num;
}
module.exports = {
  pi: 3.14,
  squareTwo: 1.41,
  phi: 1.61,
  absolute,
};

Then these files can be imported via a require statement:

const maths = require("maths");
maths.pi;
      
any

Or you can simplify a bit using the destructuring feature in JavaScript:

const { squareTwo } = require("maths");
squareTwo;
   
const squareTwo: any

CommonJS and ES Modules interop

There is a mis-match in features between CommonJS and ES Module because ES Modules only support having the default export as an object, and never as a function. TypeScript has a compiler flag to reduce the friction between the two different sets of constraints with esModuleInterop.

TypeScript’s Module Resolution Options

Module resolution is the process of taking a string from the import or require statement, and determining what file that string refers to.

TypeScript includes two resolution strategies: Classic and Node. Classic, the default when the compiler flag module is not commonjs, is included for backwards compatibility. The Node strategy replicates how Node.js works in CommonJS mode, with additional checks for .ts and .d.ts.

There are many TSConfig flags which influence the module strategy within TypeScript: moduleResolution, baseUrl, paths, rootDirs.

For the full details on how these strategies work, you can consult the Module Resolution.

TypeScript’s Module Output Options

There are two options which affect the emitted JavaScript output:

• target which determines which JS features are downleveled (converted to run in older JavaScript runtimes) and which are left intact
• module which determines what code is used for modules to interact with each other

Which target you use is determined by the features available in the JavaScript runtime you expect to run the TypeScript code in. That could be: the oldest web browser you support, the lowest version of Node.js you expect to run on or could come from unique constraints from your runtime - like Electron for example.

All communication between modules happens via a module loader, the compiler flag module determines which one is used. At runtime the module loader is responsible for locating and executing all dependencies of a module before executing it.

For example, here is a TypeScript file using ES Modules syntax, showcasing a few different options for module:

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

ES2020

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

CommonJS

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.twoPi = void 0;
const constants_js_1 = require("./constants.js");
exports.twoPi = constants_js_1.valueOfPi * 2;

UMD

(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.js"], factory);
    }
})(function (require, exports) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    exports.twoPi = void 0;
    const constants_js_1 = require("./constants.js");
    exports.twoPi = constants_js_1.valueOfPi * 2;
});

Note that ES2020 is effectively the same as the original index.ts.

You can see all of the available options and what their emitted JavaScript code looks like in the TSConfig Reference for module.

TypeScript namespaces

TypeScript has its own module format called namespaces which pre-dates the ES Modules standard. This syntax has a lot of useful features for creating complex definition files, and still sees active use in DefinitelyTyped. While not deprecated, the majority of the features in namespaces exist in ES Modules and we recommend you use that to align with JavaScript’s direction. You can learn more about namespaces in the namespaces reference page.