Angular Change Detection Illustrated

Alain Chautard

I see a lot of confusion around Angular change detection. I compared the OnPush and Default strategies in a past post, but let’s illustrate them with visuals.

Default Change Detection

An event happening anywhere in the DOM tree will have Angular check the entire tree for changes:

OnPush Change Detection

If a hierarchy of components is using OnPush, then only that branch on OnPush components will be checked for changes:

Signal Change Detection

This is the future of Angular. When using Signals, only the views of components that use that Signal will get updated, making it the best and most accurate option:

Creating a library with Angular

Alain Chautard

You’re probably used to creating Angular applications, but what about Angular libraries?

First, it’s essential to define what an Angular library is. A library is a collection of components/pipes/directives or services bundled together that can be shared and used in multiple code repositories. As a result, Angular libraries are designed to be published on npm (publicly or privately) so they can be shared with other people externally or internally.

If you want to start a new library from scratch, these Angular CLI commands will get it done:

The above commands create a projects/my-lib folder in the workspace, with a sample component and service in it. The main difference between a library and an application is that a library exposes public features that can be imported into other libraries or applications.

Such features are listed and exported in public-api.ts in the library folder. That’s where you decide what’s public/private in your library code. In this example, the library is just one service:

Then, to test or build your library, you can use regular Angular CLI commands such as:

Once built, a library can be published to the public npm repository with one single command. This command has to be run from the dist folder where the compiled library can be found after running your production build:

Note that this command requires an npm account and npm authentication before you can publish. Upon publishing, the version number used in your package.json will be used as the public version number on npm:

And on npm’s website:

At that point, anyone can run npm install [your-library-name] and use your code in their projects. Nice and easy!

Note that the library I used in that example is a work in progress and should not be used as-is in your apps.

Angular 17 is coming soon…

Alain Chautard

Angular 17 tentative release date is set for November 8, 2023. Be ready for a few good surprises along the way, but for now, I can share that the new control flow features will be available with that version.

There was a control flow RFC a few months back. The Angular team received valuable feedback from it and decided to go with the following syntax:

The above syntax can be used instead of ngIf and its awkward else syntax. Also visible in the above example is @defer, a way to lazy-load a component based on different criteria.

Of course, we’ll cover all this in the newsletter in November once all these features are officially released. In the meantime, mark your calendar: November 8 is the date.

Backticks, double quotes, and single quotes

Alain Chautard

When working with strings in Javascript, we have three options: Backticks, double quotes, and single quotes. Today, let’s look at when to use each of these and the pros and cons of each approach.

Single-quotes

Single quotes are the least exciting option because they have primarily cons:

Double-quotes

Double quotes get the job done most of the time but don’t shine when building template strings with multiple lines:

Backticks

Backticks are superior most of the time because they were designed to make template strings easy:

It’s been proven that there are no differences in performance when using backticks in modern Javascript. It’s also important to remember that with Angular, our TypeScript compiler will compile those backticks into double quotes if our compiler is set to any version of Javascript before ES6, so you don’t have to worry about browser compatibility either. All modern browsers support template literals with backticks.

Fine-tuning your eslint configuration

Alain Chautard

Last week, we introduced eslint and how it can help improve our code by identifying places where we have dead code or don’t follow best practices. Sometimes, we “break” some of these rules on purpose or decide to adopt a different convention, which is perfectly fine.

In that case, instead of giving up on eslint entirely, a better idea is to change its configuration to tweak the severity of a rule or even disable it. An es lint rule has three different severity settings:

  • “off” or 0 – turns the rule off
  • “warn” or 1 – turns the rule on as a warning (doesn’t make the lint command fail)
  • “error” or 2 – turns the rule into an error (makes the lint command fail with exit code 1 – a good option to fail a continuous integration build)

Such severity tweaks can be made in the .eslintrc.json file created in your project by the Angular schematics:

In the above example, I made the first two rules throw an error instead of a warning (I’m very much against disabling type-checking in TypeScript), but I’m OK with seeing some var keywords instead of let, so I turned off that third rule.

Getting the rule’s name is easy: When the linter fails, that name will be displayed in the console. Here @typescript-eslint/no-empty-function :

Some rules accept more configuration options to create an allowlist of accepted values. For instance, @angular-eslint/no-input-rename prevents you from renaming @Input values, but you can specify a config option that allows a few input names:

The config for that rule becomes an object that looks like this:

The above config allows renaming inputs only if the new name is check or test. This gives you more flexibility than turning off a rule entirely if it’s too restrictive for you.

Improve your code with eslint

Alain Chautard

eslint is a popular linter that parses your code and outputs a list of warnings and errors to help you improve. The library is designed to lint JavaScript code, and there are extra plugins for TypeScript and Angular so we can get even more specific feedback for our components and services. Here is an example of linting output:

A linter is a perfect complement to a compiler. For instance, angular-eslint, the eslint plugin for Angular, will also look at your HTML templates and flag code that doesn’t follow the Angular style guide. It’s also looking for possible mistakes, such as getting the ngModel 2-way data-binding syntax wrong:

To give you a better idea, here is the list of all the template rules and all the Angular TypeScript rules. If you want to give eslint a try, the first step is to install it with the help of some schematics:

This will download the proper dependencies and plugins and set up everything necessary to lint your code. If you’re using an older version of Angular or building a library instead of an app, there are step-by-step instructions to follow here. Once the set-up is done, all you have to do is run:

This command will parse all your files and output feedback in the console. Note that several IDEs can detect your eslint config and suggest automatic fixes to linting errors, which is even better!

Typescript Cheatsheet: Control Flow

Alain Chautard

A few weeks back, I shared a cheat sheet for Typescript classes. Today, I want to share a cheat sheet on control flow, which will change in Angular templates very soon.

Note the different type narrowing options, such as: if (“property” in object), which can come in handy instead of creating extra types. Type guards are exciting as well, though rarely used in Angular applications:

Click this link to see a larger version in a browser.

Directive Composition API

Alain Chautard

In the past few weeks, we’ve covered different examples of directives, such as the CDK Listbox directive and the CDK Drag and Drop directive. I have mentioned that directives are underused in Angular applications these days, and one way to use more directives is to adopt the directive composition API.

Let’s consider our previous example of a list of items:

Let’s assume we want to make our list flexible enough to support re-ordering items using drag and drop. To do so, we could start using multiple different directives on the same element, such as cdkOption and cdkDrag (code example here on Stackblitz):

While the above code works, it’s designed for a one-time use case. If we know that our application will use several droplists that support drag-and-drop, we should start thinking about creating our own custom directive that refactors these different features in one place. Enter the directive composition API:

This new directive draggableOption is composed of both cdkOption and cdkDrag to achieve the desired result. We have one input text that is forwarded to the cdkOption input thanks to this syntax:

Note that both inputs and outputs can be forwarded that way (code examples here). The beauty of this approach is that our new directive has very little code while packing reusable features in a concise syntax. This is how we would use our new directive:

You can find that example live on Stackblitz here. There are a few caveats with the directive composition API, mainly that it only works with standalone directives and that the options to “forward” inputs and outputs are limited for now. However, the composition API is an excellent option to make our code more reusable and immune to copying and pasting collections of directives from one component to another.

JavaScript rest operator

Alain Chautard

This is the final post in our JavaScript series on destructuring. Let’s focus on the rest operator, which materializes with three little dots ... as follows:

One way to put it is that the rest operator means “everything else goes into a new array.”

This syntax also works with objects, with the “rest of the properties going into a new object”:

The rest operator can also be used in a function to catch several parameters in an array:

Array destructuring with JavaScript

Alain Chautard

In our last post, we saw how to implement object destructuring. A similar feature is available for arrays and replaces the following syntax:

With destructuring, we can simplify that code into:

The only difference with object destructuring is the usage of square brackets [] instead of curly ones {}. Default values are supported, too:

And if you want to ignore one or more items in the array, you can leave blank spaces instead of variable names:

You can find more examples in the MDN web docs here.