Javascript: Threadsafe WASM-safe fetch() and Response alternative

fetch.js

{
	class Response_sync extends Response {
		#zzz
		constructor(...args) {
			super(...args);
			this.#zzz = args[0];
		}
		text() { // WASM-safe Response.textd()
			return this.#zzz;
		}
		json() { // WASM-safe Response.json()
			return JSON.parse(this.#zzz);
		}
	}
	Response = Response_sync;
}
function fetch(resource_, init_={method: 'GET', headers: new Headers()}) {
	// Firstly: make sure GIGO doesn't happen (Garbage in, garbage out.)
	let resource;
	let init;
	if (typeof(resource_)==Request) {
		resource = resource_.url;
		init = {
			method: resource_.method ?? 'GET',
			headers: resource_.headers ?? new Headers(),
			body: resource_.body ?? undefined,
			mode: resource_.mode,
			credentials: resource_.credentials,
			redirect: resource_.redirect,
			referrer: resource_.referrer,
			referrerPolicy: resource_.referrerPolicy,
			integrity: resource_.integrity,
			keepalive: resource_.keepalive,
			signal: resource_.signal
		};
	} else {
		resource = resource_;
		init = init_;
		init.method ??= 'GET';
		init.headers ??= new Headers();
		init.body ??= undefined;
	}
	resource ??= '/';
	var back = null;
	let xml = new XMLHttpRequest(); // Prepare the XML request.
	xml.open(init.method, resource, false); // Use the method, url, and make it synchronous (for WASM use.)
	for (let pair of init.headers.entries()) { xml.setRequestHeader(...pair); } // Set the headers.
	//xml.responseType = ''; // Give us back TEXT.
	xml.onreadystatechange = function() {
		if (xml.readyState!=4) return; // If we haven't downloaded everything yet, just don't do anything.
		headers = new Headers(); // hell begins
		for (let part of xml.getAllResponseHeaders().split('\r\n')) { // for each header
			part ??= ''; // if null or undefined
			if (part=='') continue; // or empty, return
			part = part.split(': '); // otherwise, parse from "key: value" into ["key", "value"]
			headers.append(...part); // and add it.
		}
		let a = document.createElement('a');
		a.href = resource;
		// Ok, we're done!
		back = new Response(
			xml.response, // Well, the... response.
			{
				'status': xml.status, // Get the status from the XML request. (Ex. 200)
				'statusText': xml.statusText, // Get the status text from the XML request. (Ex. "OK")
				'headers': headers // Good thing we set that up!
			}
		);
		Object.defineProperties(back, {'type': {'value': 'basic'}, 'url': {'value': a.protocol+'//'+a.host+a.pathname+a.search+a.hash}});
	}
	xml.send(init.body);
	return back;
}

its WASM not NASM me dumb
i added sync response so its not horrible