Perf and template syntax – Example 1

Alain Chautard

Yesterday, I sent you 3 different examples of Angular binding syntaxes and asked you to take a look at them and identify the pros and cons of each one.

Today, let’s cover the pros and cons of our first example:

Example #1 – Array of data

<div *ngFor="let data of getData()">

The only pro of this example is the simplicity of the syntax. Other than that, the syntax uses one of the anti-patterns covered earlier in this newsletter: Calling a method in a template. The problem is that this method will be called a lot more than you’d think, and if that code happened to create a new array every single time, it could be really bad in terms of performance, as Angular would have to re-render the entire list pretty much every time the user does anything in the application.

What can we do to improve it?

  1. Use a variable instead of a method – this fixes the performance issue right away:

    <div *ngFor="let data of dataList">
  2. Use a trackBy function to avoid useless re-renders. By telling Angular what’s the unique ID of each list item, Angular would be less confused, but it’s a sub-par solution:

    <div *ngFor="let data of getData(); trackBy: trackById">

Conclusion

If you want to iterate over data that’s in your component (not an observable or a Signal), then use a variable instead of calling a method. Easy enough.

Performance and template syntax

Alain Chautard

Today, I’d like to challenge you with a small exercise. Take a minute to examine the three following code examples and decide which option performs best. Note how similar those three options are from a syntax standpoint, yet they yield very different results performance-wise:

Example #1 – Array of data

<div *ngFor="let data of getData()">{{data.name}}</div>

Example #2 – Observable

<div *ngFor="let data of getData() | async">{{data.name}}</div>

Example #3 – Signal

<div *ngFor="let data of dataSignal()">{{data.name}}</div>

Feel free to list the pros and cons of each approach. You can even respond to that email with your answers if you want. Tomorrow, I’ll give you my insights.

Everything you need to know about Arrow Functions

Alain Chautard

Arrow functions have become the standard syntax in modern JavaScript. They offer a concise way to write functions, making your code lighter and more readable. Let’s explore arrow functions and their various syntax options with code examples.

Basic syntax

Two things to note:

  • Parentheses are required when using multiple function parameters.
  • In a one-liner arrow function, the result of the expression is automatically returned.

Zero or One Parameter

If your function has only one parameter, parentheses become optional. If there’s no parameter, then you do need the parentheses:

Multiple instructions

If your function has multiple lines of code, then you need curly braces and a return statement:

Why use arrow functions?

Aside from the syntax improvements described above, arrow functions are helpful in classes because they preserve the context of this. By default, in JavaScript, this means “the current function,” not “the current class instance.” Arrow functions fix that scope for us:

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.