Per avaer/window-fetch#2, I don't think adding manual options everywhere is the right resolution.

Suppose the intercept feature is done.

At that point, you'd still need to decide what User-Agent value to pass to fetch. And that decision would presumably be made somewhere other than inside the DOM element, just before the call to fetch.

In a multi-window multi-worker environment, it's not possible to know what the value of User-Agent should be for any arbitrary DOM element. The correct User-Agent value is window.navigator.userAgent. However, given DOM element X and DOM element Y, then X and Y might have completely different values for this.ownerDocument (and hence, different values for their corresponding window.navigator.userAgent).

Suppose User-Agent is a static unchanging value, knowable at startup time. Then in that case, simply setting something like

const _fetch = fetch;
fetch = (url, opts, ...args) => {
  let h = (opts && opts.headers) ? opts.headers : new Headers();
  if (!(h instanceof Headers)) { h = new Headers(h); }
  if (!h.has('User-Agent')) {
   h.set('User-Agent', `User-Agent: ${ExokitUserAgent}`);
  }
  return _fetch(url, {... (opts || {}), headers: h}, ...args);
}

at the top of src/DOM.js would give you the equivalent functionality as the intercept API. But it's ugly compared to just passing in a value.

The purpose of interception is to generalize the above case into a central control point.

That includes:

• Different fetch callpoints
• XMLHttpRequest
• User fetch
• Various Workers

Yeah, I agree with @modulesio. This is a really great idea and we should be setting the header, but it'd be better to do it inside the fetch implementation rather than in the code invoking fetch.

Closing in favor of #1237