Building Polyfills

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We used the --out-dir option above. You can view the rest of the options accepted by the cli tool by running it with --help. But the most important to us right now are --plugins and --presets. Transformations come in the form of plugins, which are small JavaScript programs that instruct Babel on how to carry out transformations to the code. You can even write your own plugins to apply any transformations you want to your code. Now any arrow functions in our code will be transformed into ES5 compatible function expressions:.

That's a good start! Instead of adding all the plugins we want one by one, we can use a "preset" which is just a pre-determined set of plugins. Just like with plugins, you can create your own presets too to share any combination of plugins you need. For our use case here, there's an excellent preset named env.

with the CLI

But presets can take options too. Rather than passing both cli and preset options from the terminal, let's look at another way of passing options: configuration files. There are a few different ways to use configuration files depending on your needs. Be sure to read our in-depth guide on how to configure Babel for more information. Now the env preset will only load transformation plugins for features that are not available in our target browsers.

We're all set for syntax. Let's look at polyfills next. This means you can use new built-ins like Promise or WeakMap , static methods like Array. The full bundle adds 94kb to your critical loading path, whereas the loader only adds 5kb. You should balance the needs of the likely minority of your users on old browsers with the convenience of the likely majority on evergreen browsers. In most cases, you'll want to load the webcomponents-loader.

But there will be times you'll want to load it asynchronously. For example: if your app implements a static app-shell to give users the illusion of performance, you'll want that static HTML and CSS to load as quickly as possible, which means eliminating render-blocking resources. In those cases, you'll need to use the window. If you're using a web component library like Polymer , LitElement , or hybrids among others to write your components something we'll cover in a later post , your components will work with the polyfills out-of-the-box, since those libraries are specifically written to use the polyfills.

Your job is done. Have a beer. But if you're writing your components without using a library first of all, good for you , you'll need to jump through a few hoops to make sure that your components render correctly for as many users as possible. Eagle-eyed readers may have noticed a few tricky lines of JavaScript peppered into one of the examples that we used in the last post:.

See that ShadyCSS reference?

Prevent Loading Redundant ES2015+ Browser Polyfills Thanks to Angular CLI

That's the part of the polyfills which emulates the style-scoping of shadow DOM in browsers which don't support it. In order for your styles to be scoped properly, there are a few rules to follow:. Because of the way the ShadyCSS polyfill works, web component authors that need to support Edge or worse are advised not to use dynamically generated CSS such as:. Those are contrived examples. In the real world you're more likely to solve the problem entirely with CSS like:.

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But if you wanted to, say, rotate a hue based on touch events or transform an element based on websocket updates, CSS Custom Properties are the way to go. ShadyCSS provides some other features like a shim for the now-deprecated apply CSS syntax, but we're not going to cover them because that spec is dead in the water.

ng build --prod gives error on · Issue # · angular/angular-cli · GitHub

There are also some known limitations to the ShadyCSS polyfill. It also exposes the customElements object on the window , so you can register your components. The polyfill upgrades custom elements after DOMContentLoaded , then initializes a MutationObserver to upgrade any custom elements that are subsequently attached with JavaScript. The polyfills support IE11, but it's not all sunshine and rainbows. IE11 is no longer developed by MS, which means it should not be used.

Deciding to support IE11 means added development time, added complexity, added surface area for bugs, and exposing users to a buggy, outdated browser. Any time IE11 support is raised as a requirement, it has to be carefully evaluated. Don't just lump it in as a "nice to have". I now have a complete unit test setup that works in the browser and via the console, which is about to come in quite handy!

Performing automated testing across several installed browsers can be a quick sanity check to ensure that development of your polyfill is progressing along without a hitch and, thankfully, there are some great tools out there that we can pair with our existing Grunt workflow.

My tool of choice is Karma , a simple test runner that can call out to all major browsers using test runner plug-ins and also integrates nicely with Grunt. Both Karma and grunt-karma will be installed, and a series of Karma-related dependencies will be added to your package. Key in this section is the configFile property, which accepts a path to a separate Karma configuration file.

Karma has a lot of configuration options, so placing these in a separate file is usually a good approach. The contents of my karma. This file contains a couple of key sections.

First, the files array contains all of the source files from my project that Karma needs to know about in order to properly execute my tests. Next, I include a series of options regarding the test runner Jasmine , which ports to use, and the like. Finally, I specify a series of Karma plug-ins, which the framework needs to communicate with my test runner and browsers.

If I do that, I should see all of my browsers launch, but one of my three initial tests will fail. Can you guess which ones?

Netanel Basal

Much as I did for Jasmine in the console, I need to add a path condition for Karma to my fixtures. As mentioned earlier, Grunt gives you the ability to create custom tasks that chain together predefined tasks in interesting ways. Now I can run grunt test in the console and have my files linted, minified, combined, and tested, or I can run grunt x-test , which will do all of the above and then run my tests across all browsers using Karma.

What's new in Angular CLI 8.0?

Linting is the practice of checking source code for syntax errors or style violations. Then, when one of those files is saved, Grunt will execute one or more tasks automatically. Sounds awesome, right? All I need is the grunt-contrib-watch plug-in, which I can install via npm:. Now I can run grunt watch before I begin working on my polyfill. In fact, all I need in my project is to add a file called. Once everything is set up, any time I push a commit to my repo from GitHub to Travis, the service will spin up and run my tests, giving me that extra measure of defense for my polyfill.

Looks like a success! However, the full kendo-ui-forms polyfill contains a much larger test suite, and I suggest you check it out for yourself when venturing into testing for your own polyfills. In this chapter, I mentioned a lot of third-party and open source technologies that I prefer to use when building polyfills. Some of these might work for you as well, while in other cases you have a personal favorite you like to use.

All that matters is that you get a good dev and test workflow in place early on, no matter which tools you choose to use. Stay ahead with the world's most comprehensive technology and business learning platform. With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more. Start Free Trial No credit card required. Setting Up Your Project Workflow.