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4.5.0 • Public • Published

The coach

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The coach

The coach helps you find performance problems on your web page. You can think of the coach as a modern version of YSlow.

Does my page need coaching?

You know, it's hard to get everything right! The world is complex: HTTP/1 vs HTTP/2. Some of the previous performance best practices are now worst practices. The coach will detect that the page is accessed using HTTP/2, and adjust its advice accordingly.

Why we love the coach

Ten reasons why we love the coach:

  • The coach gives you advice on how to make your page faster. The coach aims to NEVER give you bad advice. Follow the advice and you will WIN!
  • HTTP/1 or HTTP/2? That's no problem, the coach adjust the advice accordingly.
  • The coach uses real browsers to see your page exactly like your users do.
  • Every advice has one or more unit-tests to make sure it's easy to change advice in the future.
  • The coach knows about more than just performance: Accessibility and web best practice are other things that the coach can help you with.
  • You can integrate the coach with your own web performance tool. It's easy: your tool only need to be able to run JavaScript in the browser and produce a HAR file. Or you can use the built-in functionality of the coach to run the browser.
  • The coach is open-source. The advice is public, you can check it and change it yourself. Help us make the coach even better!
  • The coach can combine knowledge from the DOM with HAR to give you super powerful advice.
  • The CLI output is pretty nice. You can configure how much you want to see. Use it as fast way to check the performance of your page.

How to use the coach

You can use the coach in a few different ways.


docker run sitespeedio/coach

To build a docker image from source you will need to run a few commands

npm install
npm run gen-dist
docker build -t my-custom-coach .


You need Node.js LTS (you need to be on 8 or later). And you need Chrome and/or Firefox installed.

If you want to use Chrome (Chrome is default):


Try it with Firefox (needs Firefox 57 or later):

npm install webcoach -g
webcoach --browser firefox

If you also want to show the offending assets/details and the description of the advice:

webcoach --details --description

By default, the coach only tells you about advice where you don't get the score 100. You can change that. If you want to see all advice, you can do that too:

webcoach --limit 101

If you want to test as a mobile device, that's possible too, by faking the user-agent.

webcoach --mobile -b chrome

... but hey, I want to see the full JSON?

Yes, you can do that!

webcoach -f json

This will get you the full JSON, the same as if you integrate the coach into your tool.

... but hey, I don't want any color in the table output?

That's possible as well!

webcoach --noColor


We also produce a bookmarklet. The bookmarklet only uses advice that you can run inside the browser (it doesn't have HAR file to analyse even though maybe possible in the future with the Resource Timing API).

The bookmarklet is really rough right now and logs the info to the browser console. Help us make a cool front-end :)

You can generate the bookmarklet by running

npm run bookmarklet

and then you will find it in the dist folder.

Chrome Dev Tools Extension

Installing the Coach Panel Chrome Extension, will add a 'Coach' tab inside your Chrome Dev Tools from there you can analyse and view your page's results.

Like the bookmarklet it only uses advice that you can run inside the browser - just until that Resource Timing API supports sizes in Chrome!

Alternatively view the Source Code.

The coach in dev tools

Thank you Matt for creating the extension!

Include in your own tool

The coach uses Browsertime to start the browser, execute the JavaScript and fetch the HAR file. You can use that functionality too inside your tool or you can use the raw scripts if you have your own browser implementation.

Use built in browser support

In the simplest version you use the default configuration (default DOM and HAR advice and using Firefox):

const api = require('webcoach');
const result ='');

The full API method:

// get the API
const api = require('webcoach');
const result =, domScript, harScript, options);

Use the scripts

Say that your tool run on Windows, you start the browsers yourself and you generate your own HAR file. Create your own wrapper to get the coach to help you.

First you need the JavaScript advice, you can get the raw script either by generating it yourself or through the API.

Generate the script

npm run combine

and it will be in the dist folder.

Or you just get it from the API:

// get the API
const api = require('webcoach');
// get the DOM scripts, it's a promise
const domScriptPromise = api.getDomAdvice();

Take the domScript and run it in your browser and take care of the result.

To test the HAR you need to generate the HAR yourself and then run it against the advice.

const api = require('webcoach');
// You read your HAR file from disk or however you get hold of it
const harJson = //
// if your har contains multiple pages (multiple runs etc) you can use the API
// to get the page that you want
const firstPageHar = api.pickAPage(harJson, 0);
// the result is a promise
const myHarAdviceResultPromise = api.runHarAdvice(firstPageHar, api.getHarAdvice());

When you got the result from both the DOM and the HAR you need to merge the result to get the full coach result:

// Say that you got the result from the browser in domAdviceResult
// and the HAR result in harAdviceResult
const coachResult = api.merge(domAdviceResult, harAdviceResult);

Now you have the full result (as JSON) as a coachResult.

What does the coach do

The coach will give you advice on how to do your page better. You will also get a score between 0-100. If you get 100 the page is great, if you get 0 you can do much better!

How does it all work?

The coach tests your site in two steps:

  • Executes JavaScript in your browser and check for performance, accessibility, best practice and collect general info about your page.
  • Analyze the HAR file for your page together with relevant info from the DOM process.

You can run the different steps standalone but for the best result run them together.

What the coach does


The coach knows more than just performance. She also knows about accessibility and web best practice.


Make sure your site is accessible and usable for everyone. You can read more about making the web accessible here.

Best practice

You want your page to follow best practices, right? Making sure your page is set up for search engines, have good URL structure and so on.


Make sure that your page respect user privacy.

General information

The world is complex. Some things are great to know but hard for the coach to give advice about.

The coach will then just tell you how the page is built and you can draw your own conclusions if something should be changed.


The coach has a clock and knows how to use it! You will get timing metrics and know if you are doing better or worse than the last run.

Developers guide

Checkout the developers guide to get a better feeling how the coach works.

Browser support

The coach is automatically tested in latest Chrome and Firefox. To get best results you need Chrome or Firefox 48 (or later) to be able to know if the server is using HTTP/2.

We hope that the coach works in other browsers but we cannot guarantee it right now.


npm i webcoach

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