Why is there a limitation on exporting an interface by default in TypeScript?

General Discussions
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1

Why is there a limitation on exporting an interface by default in TypeScript?

I’m using TypeScript 1.5 beta, and when attempting to export an interface as the default export, the following code causes an error in both Visual Studio and WebStorm:

export default interface Foo {...}

However, the following code works correctly:

interface Foo {...}
export default Foo;

Is this behavior by design, a bug, or am I doing something wrong?

EDIT: Thank you for your answer. This raises another question: what is the recommended way to import an interface using the ES6 module syntax?

This works:

// Foo.ts
export interface Foo {}

// Bar.ts
import { Foo } from 'Foo'; // Notice the curly braces

class Bar {
    constructor(foo: Foo) {}
}

But, since this works, why not allow default exports for interfaces and avoid using curly braces?

// Foo.ts
export default interface Foo {}

// Bar.ts
import Foo from 'Foo'; // Notice, no curly braces!

class Bar {
    constructor(foo: Foo) {}
}

typescript export interface: Why is the default export of interfaces restricted, and what is the recommended approach for using them with import/export syntax?

2

Hello,

When working with TypeScript, it’s essential to avoid default exports for interfaces since TypeScript does not directly support them. Instead, using named exports for interfaces ensures proper compatibility and avoids potential issues.

Here’s a simple example:

// Foo.ts
export interface Foo {
  // properties
}

// Bar.ts
import { Foo } from './Foo'; // Using named imports

class Bar {
  constructor(foo: Foo) {}
}

This approach is aligned with the ES6 module system and is the recommended method for exporting interfaces in TypeScript.

Best regards

3

Hello Community!

When working with TypeScript, you might notice that interfaces cannot be exported as default exports. However, there’s a simple solution to this: use a type alias instead. Type aliases can be exported as default, making the import process smoother and eliminating the need for curly braces.

Here’s how you can do it:

In Foo.ts:

type Foo = {
  // properties
};
export default Foo;

In Bar.ts:

import Foo from './Foo'; // No curly braces needed!

class Bar {
  constructor(foo: Foo) {}
}

This approach lets you follow the default export pattern effortlessly, avoiding the limitations of exporting interfaces by default. It’s a clean and effective way to maintain simplicity in your TypeScript code.

Thank you for reading, and happy coding! :blush:

4

Export Interface After Declaration: Another alternative is to declare the interface first and then export it, as TypeScript allows exporting interfaces this way.

// Foo.ts interface Foo { // properties } export default Foo;

// Bar.ts import Foo from ‘./Foo’; // No curly braces needed!

class Bar { constructor(foo: Foo) {} } This method adheres to TypeScript’s behavior of exporting interfaces and still allows you to import the interface with default export, as long as the typescript export interface pattern is correctly followed.