[whatwg-fetch] requires ES6 target or node.d.ts but doesnt need to

[danmortimer]

the whatwg-fetch typings reference the interfaces Symbol and IterableIterator so if you target ES5 it fails to build. The fetch project it matches at https://github.com/github/fetch has built in fallbacks for the iterators and is meant to be used with ES5. So it shouldnt reference those ES6 types.
I cant use the typings as they are without including core-js or node.d.ts or targeting ES6, which is not actually required by the library itself.

[vvakame]

please send a pull request. I'll review it.

[tobico]

Looks like the problem lines are these ones, defining an iterator interface for the Headers class - https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/whatwg-fetch/whatwg-fetch.d.ts#L24-L28

These methods do exist, but they're not essential to use the library, and the type they rely on doesn't exist without ES6.

Anyone know of a way to make these definitions conditional on the types actually existing? Or any other nice way to handle this without breaking things for either use case?

[vvakame]

DefinitelyTyped/node/node.d.ts

Lines 21 to 27 in ebfa2f0

	// compat for TypeScript 1.8
	// if you use with --target es3 or --target es5 and use below definitions,
	// use the lib.es6.d.ts that is bundled with TypeScript 1.8.
	interface MapConstructor { }
	interface WeakMapConstructor { }
	interface SetConstructor { }
	interface WeakSetConstructor { }

We can use this pattern.

[seansfkelley]

The workaround I used for this was to install the typings for es6-promise and then add these to a miscellaneous/catch-all "extra typings" file I have around:

type IterableIterator<T> = any;
type IteratorResult<T> = any;
declare const Symbol: {
  iterator: symbol;
};

I wasn't able to find all of these typings shimmed out elsewhere, so I just basically exempted the whole from type checking.

[danmortimer]

tobico:

These methods do exist, but they're not essential to use the library, and the type they rely on doesn't exist without ES6.

That actually isn't really the case.
Check this: https://github.com/github/fetch/blob/master/fetch.js#L40-L56

If it sees that Symbol.iterator is supported (either es6 or a polyfill) then it uses those types. Otherwise it shims in the basic iterator functionality.

Because of this, I think the best option is to type the iterator shim.
That could be combined with the pattern suggested by vvakame, and set those methods to return union types so it can use either the ES6 type or the shim.

[orta]

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