Removing jQuery from GitHub.com frontend 

Web standards in the later years

Over the years, GitHub grew into a company with hundreds of engineers and a dedicated team gradually formed to take responsibility for the size and quality of JavaScript code that we serve to web browsers. One of the things that we’re constantly on the lookout for is technical debt, and sometimes technical debt grows around dependenices that once provided value, but whose value dropped over time.

When it came to jQuery, we compared it against the rapid evolution of supported web standard in modern browsers and realized:

• The $(selector) pattern can easily be replaced with querySelectorAll();
• CSS classname switching can now be achieved using Element.classList;
• CSS now supports defining visual animations in stylesheets rather than in JavaScript;
• $.ajax requests can be performed using the Fetch Standard;
• The addEventListener() interface is stable enough for cross-platform use;
• We could easily encapsulate the event delegation pattern with a lightweight library;
• Some syntactic sugar that jQuery provides has become reduntant with the evolution of JavaScript language.

Furthermore, the chaining syntax didn’t satisfy how we wanted to write code going forward. For example:

$('.js-widget')
  .addClass('is-loading')
  .show()

This syntax is simple to write, but to our standards, doesn’t communicate intent really well. Did the author expect one or more js-widget elements on this page? Also, if we update our page markup and accidentally leave out the js-widget classname, will an exception in the browser inform us that something went wrong? By default, jQuery silently skips the whole expresion when nothing matched the initial selector; but to us, such behavior was a bug rather than a feature.

 

Finally, we wanted to start annotating types with Flow to perform static type checking at build time, and we concluded推断 that the chaining syntax doesn’t lend itself well to static analysis, since almost every result of a jQuery method call is of the same type. We chose Flow over alternatives because, at the time, features such as @flow weak mode allowed us to progressively and efficiently start applying types to a codebase which was largely untyped.

All in all, decoupling from jQuery would mean that we could rely on web standards more, have MDN web docs be de-facto实际上的 default documentation for our frontend developers, maintain more resilient code in the future, and eventually drop a 30 kB dependency from our packaged bundles, speeding up page load times and JavaScript execution times.

 

Incremental decoupling

Even with an end goal in sight, we knew that it wouldn’t be feasible to just allocate all resources we had to rewriting everything from jQuery to vanilla JS. If anything, such a rushed endeavor would likely lead to many regressions in site functionality that we would later have to weed out. Instead, we:

• Set up metrics that tracked ratio of jQuery calls used per overall line of code and monitored that graph over time to make sure that it’s either staying constant or going down, not up.

  

• We discouraged importing jQuery in any new code. To facilitate that using automation, we created eslint-plugin-jquery which would fail CI checks if anyone tried to use jQuery features, for example $.ajax.
• There were now plenty of violations of eslint rules in old code, all of which we’ve annotated with specific eslint-disable rules in code comments. To the reader of that code, those comments would serve as a clear signal that this code doesn’t represent our current coding practices.
• We created a pull request bot that would leave a review comment on a pull request pinging our team whenever somebody tried to add a new eslint-disable rule. This way we would get involved in code review early and suggest alternatives.
• A lot of old code had explicit coupling to external interfaces of pjax and facebox jQuery plugins, so we’ve kept their interfaces relatively the same while we’ve internally replaced their implementation with vanilla JS. Having static type checking helped us have greater confidence around those refactorings.
• Plenty of old code interfaced with rails-behaviors, our adapter for the Ruby on Rails approach to “unobtrusive” JS, in a way that they would attach an AJAX lifecycle handler to certain forms: 

 

Polyfills

These are the polyfills that helped us transition to using standard browser features. We try to serve most of these polyfills only when absolutely necessary, i.e. to outdated browsers as part of a separate “compatibility” JavaScript bundle.

• github/eventlistener-polyfill
• github/fetch
• github/form-data-entries
• iamdustan/smoothscroll
• javan/details-element-polyfill
• jonathantneal/closest
• kumarharsh/custom-event-polyfill
• marvinhagemeister/request-idle-polyfill
• mathiasbynens/Array.from
• mathiasbynens/String.prototype.codePointAt
• mathiasbynens/String.prototype.endsWith
• mathiasbynens/String.prototype.startsWith
• medikoo/es6-symbol
• nicjansma/usertiming.js
• rubennorte/es6-object-assign
• stefanpenner/es6-promise
• webcomponents/template
• webcomponents/URL
• webcomponents/webcomponentsjs
• WebReflection/url-search-params
• yola/classlist-polyfill