ES6 Modules default exports interop with CommonJS

[teppeis]

CommonJS cannot requires a package transpiled from TypeScript ES6 Modules using default export well.

TS 1.5-alpha transpiles default exports:

// foo.ts
export default foo;

to:

// foo.js ("main" in this package.json)
exports.default = foo;

So users of this package foo need to add .default property to require().

var foo = require('foo').default;

It's a little messy...

Babel resolves this issue.

http://babeljs.io/docs/usage/modules/#interop

Interop

In order to encourage the use of CommonJS and ES6 modules, when exporting a default
export with no other exports module.exports will be set in addition to exports["default"].

export default test;

exports["default"] = test;
module.exports = exports["default"];

If you don't want this behaviour then you can use the commonStrict module formatter.

It sounds pretty nice and actually works for me.

[ahejlsberg]

We considered that, but it only works if you also emit an __esModule marker property and emit a dynamic check for such a marker on every default import (which is what Babel does). Since you can already get the desired effect with TypeScript's existing export =, and since you can import that with the existing import x = require(...) syntax, we felt that it was best to stick with pure ES6 module semantics and keep the code generation as clean as possible.

Adding a bit more detail, Babel's compatibility mode generates a default export as an assignment to module.exports only when the module exports nothing but a default. The minute you add any other export, the default export now "snaps back" and becomes an export named default. Hence you need an __esModule marker to indicate which mode the module is in, and you need a dynamic check of that marker on every import. It gets rather messy.

[mizchi]

This behaviour is my blocker to adopt 1.5.
I expected module.exports = with --module commonjs when I use export default but TypeScript didn't so.

export default class Foo {}

commonjs users have to write below now with 1.5. (not for typescript, raw js for node.js)

let Foo = require('./foo').default

But this problem may be common.js/require spec's...

*added * I wrote typescript as a part of my project, and provide common.js module to be required.

[ahejlsberg]

@mizchi You're right, if you use export default then CommonJS consumers will have to access the default property. This is the one big difference between default exports in ES6 and assigning to module.exports in CommonJS. There is really no clean way to solve this problem. Babel supports a compatibility mode and a magic __esModule marker that you dynamically have to check, but I'm not convinced that's any better. My feeling is we're better off just going with pure ES6 semantics.

[teppeis]

@ahejlsberg yes, there is no clean way. I see your policy for interoperability with CommonJS.

So, how about interop with other ES6 Module transpilers?
Babel, Traceur, SystemJS and some others cannot import a module from TypeScript 1.5-alpha as default properly.

// foo.ts (main of package "foo")
export default 'foo';

// An ES6 module to be compiled by Babel, Traceur, SystemJS
import foo from 'foo';
console.log(foo); // { default: 'foo' } instead of just 'foo'

If TypeScript sets exports.__esModule = true as a transpiled ES6 module, they import it as a default.

[vvakame]

👍

[niemyjski]

+1

[scottwio]

+1

[okunokentaro]

+1

[EddyLane]

+1

[vladkosinov]

+1