Tips for exporting/importing only a type definition in TypeScript:

Is it possible to export and import a type definition separately from the module in question?

In Flowtype, achieving this can be done by having the file sub.js export the type myType with

export type myType = {id: number};
, and then in the file main.js, importing the type like so:
import type {myType} from './sub.js';

Answer №1

In Typescript versions 3.8 and above, there are now options for type-only exports and imports:

import type { AnotherThing } from "./another-module.ts";

export type { AnotherThing };

Answer №2

When you import a module normally, the compiler can figure out that there is no need to emit an import statement because no concrete code is being used.

For 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 would look like this in ES2015:

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

Alternatively, 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 key point is that the TypeScript compiler recognizes that the import statement is only necessary during compilation and not at runtime.

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