URLSearchParams/Headers constructor also receives iterables including Map

TypeScript Version: master

Code

new Headers(new Map([['abc', 'bcd']])).get('abc')
new Headers({ [Symbol.iterator]() { return new Map([['abc', 'bcd']]).entries() } }).get('abc')

Expected behavior: Both returns bcd so should pass without errors

Actual behavior: 'not assignable' error

I think this will do the work for ES2015+:

type Record<K, V> = { [Symbol.iterator](): IterableIterator<[K, V]> } | { [x: K]: V };

But we cannot make this compatible with ES5 as DOM constructor types cannot be extended.

PS: (This is wrong, the sequence type should be processed instead)

I guess if you want this to work everywhere, you need:

a new global type for Headers constructor
an overload in lib.dom.iterable.d.ts to add overloads with Symbol.Iterator on them.

URLSearchParams constructor: add overload to support iterables

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