Modules

A module is just a file — anything you export from it becomes available to other files that import it. TypeScript's module syntax is the same ES module syntax modern JavaScript already uses; the compiler adds type checking across those import/export boundaries on top.

Named exports

export function add(a: number, b: number): number {
  return a + b;
}

export const PI = 3.14159;

import { add, PI } from "./math";

console.log(add(2, 3));
console.log(PI);

5
3.14159

Each name has to be exported explicitly with export, and imported with the exact same name in curly braces — import { add } pulls in only add, not everything the file exports.

Default exports

class Logger {
  log(message: string) {
    console.log(`[LOG] ${message}`);
  }
}

export default Logger;

import Logger from "./logger";

const logger = new Logger();
logger.log("Server started");

[LOG] Server started

A file can have at most one export default — the importing file can then give it whatever local name it wants (Logger here, but it isn't required to match), unlike named exports which must be imported by their exact declared name.

Importing types

import type { User } from "./user";

function greet(user: User) {
  console.log(`Hi, ${user.name}`);
}

import type makes it explicit that only the type is being imported, not any runtime value — the compiler strips it out entirely from the compiled JavaScript, since types don't exist once compilation is done.