Response Header.get() implementaion is wrong

[giy-debug]

The Response Headers implementation is wrong. The Headers.get function converts arrays to strings. This neither conforms to Map nor the URLSearchParams class definitions. This is a problem when you try to parse the set-cookie headers

/**
  * Return combined header value given name
  *
  * @param   String  name  Header name
  * @return  Mixed
  */
	get(name) {
		name = `${name}`;
		validateName(name);
		const key = find(this[MAP], name);
		if (key === undefined) {
			return null;
		}

		return this[MAP][key].join(', '); //this line is wrong
	}

Expected behavior, if applicable

Map implementation

const map = new Map();
map.set('set-cookie', ['cookie1','cookie2'])
map.get('set-cookie'); 
// ['cookie1','cookie2']

The Headers class looks like a re-implementaion of URLSearchParams class, which is supported natively. You could save yourself a lot of code, and the implementation could match the standard.

const searchParams = new URLSearchParams();
searchParams.append('set-cookie', 'cookie1');
searchParams.append('set-cookie', 'cookie2');
searchParams.get('set-cookie');      //"cookie1"
searchParams.getAll('set-cookie');  //["cookie1", "cookie2"]

As you can see searchParams.get('set-cookie') doesn't join the values but Headers.getAll doesn't exist in your implementation.

Your Environment

software	version
node-fetch	2.6.0
node	12.18.2
npm	6.14.5
Operating System	Win10

From MDN

https://developer.mozilla.org/de/docs/Web/API/Headers

Obsolete methods

Headers.getAll()
Used to return an array of all the values of a header within a Headers object with a given name; this method has now been deleted from the spec, and Headers.get() now returns all values of a given name instead of just the first one

[jimmywarting]

Did you forgot to read the part of where it says: "Obsolete"?

Used to return an array of all the values of a header within a Headers object with a given name; this method has now been deleted from the spec, and Headers.get() now returns all values of a given name instead of just the first one.

[jimmywarting]

I agree it's somewhat problematic. but we try to follow the spec
workaround: #251 (comment)

[giy-debug]

But the join is still wrong. And it also says Headers.get() now returns all values of a given name

[jimmywarting]

But the join is still wrong. And it also says Headers.get() now returns all values of a given name

Did MDN clearly say how to return it? no, there is no mentioning of array or string in that sentence.
But one thing is for certain: They also mention elsewhere that it's returning a string...

Headers.get()
Returns a ByteString sequence of all the values of a header within a Headers object with a given name.

The official spec mention it also:

To get a name name from a header list list, run these steps:

1. If list does not contain name, then return null.
2. Return the values of all headers in list whose name is a byte-case-insensitive match for name, separated from each other by 0x2C 0x20, in order.

https://fetch.spec.whatwg.org/#terminology-headers

0x2C === comma
0x20 === space

You can test out the browser behavior in this fiddle: https://jsfiddle.net/zL04o17w/ (same thing)

[jimmywarting]

This might be relevant for ya: whatwg/fetch#973