TypeScript Version: 2.6.1 (and 2.7.0-dev.20171101)

Code

https://www.typescriptlang.org/play/index.html#src=fetch(%27%2Fsome-url%27%2C%20%7B%20headers%3A%20%7B%20%27Some-Header-Name%27%3A%20%27Value%27%20%7D%20%7D)%3B

fetch('/some-url', { headers: { 'Some-Header-Name': 'Value' } });

Expected behavior:
No errors.

Actual behavior:
Error:

Object literal may only specify known properties, and ''Some-Header-Name'' does not exist in type 'Headers | string[][]'.

According the spec RequestInit may has headers prop as HeadersInit type which is

type HeadersInit = [string, string][] | { [key: string]: string };

But currently we have Headers instead of map object in RequestInit.headers (the code above works fine in all browsers (including Edge)).

Also HeadersInit is not used in RequestInit.