Fetch Tracker

Wraps the fetch function so it can emit a few custom fetch events, useful for tracking analytics from fetch.

The events are non-destructive and will not change anything about the fetch itself (unlike the FetchEvent fetch event).

Installation

Include before you call fetch, and it will be wrapped automatically.

Important Note: If you are using a fetch polyfill, you should include this module after any polyfills or other fetch changing modules.

ES6

import 'fetch-tracker';

Common JS

require('fetch-tracker');

Browser

Minified:

<script href="path/to/fetch-tracker/dist/fetch-tracker.min.js"></script>

Unminified:

<script href="path/to/fetch-tracker/dist/fetch-tracker.js"></script>

There are also accompanying maps fetch-tracker.min.js.map and fetch-tracker.js.map.

Basic Usage

addEventListener('preFetch', event => {
  console.log(event.request);
});

addEventListener('postFetch', event => {
  console.log(event.request, event.response, event.duration);
});

fetch('http://example.com');

Two events are available, preFetch and postFetch. Add an event listener for them (at the top-level). They will be dispatched every time fetch is called (unless paused with fetch.pauseTracking()).

Properties

fetch.trackingPaused

Boolean. Indicates if tracking has been paused.

Methods

fetch.pauseTracking()

Calling this will pause any tracking of fetch, so it will not emit preFetch or postFetch events any more.

This is useful if you are going to make an especially large call that would perform poorly when cloning the Response object.

fetch.resumeTracking()

Resumes tracking after calling fetch.pauseTracking().

Events

preFetch

This event is fired before the normal fetch event.

It will receive a PreFetchEvent event, which includes the request.

The PreFetchEvent has a request property which is a clone of the original Request.

addEventListener('preFetch', event => {
  console.log(event.request.method);
});

postFetch

This event is fired once the fetch itself is complete, but before any then() functions are fired from the fetch.

It will receive a PostFetchEvent event, which includes the following properties:

• request - A clone of the original Request object.
• response - A clone of the Response object.
• duration - The time (in milliseconds) between the initiation of the fetch and when it concludes.

Note: If there are multiple postFetch events, they will share the same response, so reading the body once will mean subsequent events can't be read. If you are doing this, you should use response.clone() in at least one of those so you can read it multiple times (or combine it into one event).

addEventListener('postFetch', event => {
  event.response.json().then(json => console.log(json));
});

fetch

This event isn't implemented by this library, but if you are using a library/environment which does use it, it occurs after preFetch but before postFetch. It is unaffected by this library. MDN FetchEvent Documentation