How do ES6 modules work in TypeScript?

A file with a top-level import or export is an ES6 module with its own scope. export makes a name visible to other files, and import brings it in. A file with neither is treated as a global script instead.

// math.ts
export function add(a: number, b: number): number {
  return a + b;
}
export function subtract(a: number, b: number): number {
  return a - b;
}
export default function multiply(a: number, b: number): number {
  return a * b;
}

export const PI = 3.14159;

// app.ts
import multiply, { add, subtract, PI } from './math';
import * as MathUtils from './math';

console.log(add(5, 3)); // 8
console.log(multiply(4, 2)); // 8
console.log(MathUtils.PI); // 3.14159

A module can have any number of named exports, imported in braces. It has at most one default export, imported without braces under any name you choose. import * as MathUtils gathers every named export into one object.

TypeScript adds type-only imports. import type { User } from './types' is removed from the output, so it never loads the module at runtime. The module and moduleResolution settings in tsconfig.json decide which syntax the compiler emits and how it finds ./math.