How to export/import a only a type definition in TypeScript

Question

How can I export and import a type definition independently from the module itself.

In flowtype this would look like this: The file sub.js export the type myType using export type myType = {id: number}; and the file main.js imports the type using import type {myType} from './sub.js';

Answer 1

You just import it normally and the compiler works out that you don't need the import statement emitted because no concrete code is used.

Here's an example:

component.ts

export interface MyInterface {
    name: string;
}

app.ts

import { MyInterface } from './component';

class MyClass implements MyInterface {
    constructor(public name: string) { }
}

The app.js file is simply (ES2015 version):

class MyClass {
    constructor(name) {
        this.name = name;
    }
}

Or in older ES5 terms:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
var MyClass = /** @class */ (function () {
    function MyClass(name) {
        this.name = name;
    }
    return MyClass;
}());

The important thing here is that the TypeScript compiler has worked out that the import is only needed at compile time, not runtime.

Answer 2

Typescript 3.8 and later has type only exports and imports:

https://devblogs.microsoft.com/typescript/announcing-typescript-3-8/#type-only-imports-exports

import type { SomeThing } from "./some-module.js";

export type { SomeThing };