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Angular - Launching your app with a root module

Launching your app with a root module link

Prerequisites link

A basic understanding of the following:

  • JavaScript Modules vs. NgModules

An NgModule describes how the application parts fit together. Every application has at least one Angular module, the root module, which must be present for bootstrapping the application on launch. By convention and by default, this NgModule is named AppModule .

When you use the Angular CLI command ng new to generate an app, the default AppModule looks like the following:

      /* JavaScript imports */
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';

/* the AppModule class with the @NgModule decorator */
  declarations: [
  imports: [
  providers: [],
  bootstrap: [AppComponent]
export class AppModule { }

After the import statements is a class with the @NgModule decorator.

The @NgModule decorator identifies AppModule as an NgModule class. @NgModule takes a metadata object that tells Angular how to compile and launch the application.

metadata object Details
declarations This application's lone component.
imports Import BrowserModule to have browser-specific services such as DOM rendering, sanitization, and location.
providers The service providers.
bootstrap The root component that Angular creates and inserts into the index.html host web page.

The default application created by the Angular CLI only has one component, AppComponent , so it is in both the declarations and the bootstrap arrays.

The declarations array link

The module's declarations array tells Angular which components belong to that module. As you create more components, add them to declarations .

You must declare every component in exactly one NgModule class. If you use a component without declaring it, Angular returns an error message.

The declarations array only takes declarables. Declarables are components, directives, and pipes. All of a module's declarables must be in the declarations array. Declarables must belong to exactly one module. The compiler emits an error if you try to declare the same class in more than one module.

These declared classes are visible within the module but invisible to components in a different module, unless they are exported from this module and the other module imports this one.

An example of what goes into a declarations array follows:

      declarations: [

A declarable can only belong to one module, so only declare it in one @NgModule . When you need it elsewhere, import the module that contains the declarable you need.

Using directives with @NgModule link

Use the declarations array for directives. To use a directive, component, or pipe in a module, you must do a few things:

  1. Export it from the file where you wrote it.
  2. Import it into the appropriate module.
  3. Declare it in the @NgModule declarations array.

Those three steps look like the following. In the file where you create your directive, export it. The following example, named ItemDirective is the default directive structure that the CLI generates in its own file, item.directive.ts :

      import { Directive } from '@angular/core';

  selector: '[appItem]'
export class ItemDirective {
// code goes here
  constructor() { }


The key point here is that you have to export it, so that you can import it elsewhere. Next, import it into the NgModule , in this example app.module.ts , with a JavaScript import statement:

      import { ItemDirective } from './item.directive';

And in the same file, add it to the @NgModule declarations array:

      declarations: [

Now you could use your ItemDirective in a component. This example uses AppModule , but you'd do it the same way for a feature module. For more about directives, see Attribute Directives and Structural Directives. You'd also use the same technique for pipes and components.

Remember, components, directives, and pipes belong to one module only. You only need to declare them once in your application because you share them by importing the necessary modules. This saves you time and helps keep your application lean.

The imports array link

The module's imports array appears exclusively in the @NgModule metadata object. It tells Angular about other NgModules that this particular module needs to function properly.

src/app/app.module.ts (excerpt)
      imports: [

This list of modules are those that export components, directives, or pipes that component templates in this module reference. In this case, the component is AppComponent , which references components, directives, or pipes in BrowserModule , FormsModule , or HttpClientModule . A component template can reference another component, directive, or pipe when the referenced class is declared in this module, or the class was imported from another module.

The providers array link

The providers array is where you list the services the application needs. When you list services here, they are available app-wide. You can scope them when using feature modules and lazy loading. For more information, see Providers.

The bootstrap array link

The application launches by bootstrapping the root AppModule , which is also referred to as an entryComponent . Among other things, the bootstrapping process creates the component(s) listed in the bootstrap array and inserts each one into the browser DOM.

Each bootstrapped component is the base of its own tree of components. Inserting a bootstrapped component usually triggers a cascade of component creations that fill out that tree.

While you can put more than one component tree on a host web page, most applications have only one component tree and bootstrap a single root component.

This one root component is usually called AppComponent and is in the root module's bootstrap array.

In a situation where you want to bootstrap a component based on an API response, or you want to mount the AppComponent in a different DOM node that doesn't match the component selector, please refer to ApplicationRef.bootstrap() documentation.

More about Angular Modules link

For more on NgModules you're likely to see frequently in applications, see Frequently Used Modules.

Last reviewed on Mon Feb 28 2022
Angular - Browser support

Browser support link

Angular supports most recent browsers. This includes the following specific versions:

Browser Supported versions
Chrome latest
Firefox latest and extended support release (ESR)
Edge 2 most recent major versions
Safari 2 most recent major versions
iOS 2 most recent major versions
Android 2 most recent major versions

Angular's continuous integration process runs unit tests of the framework on all of these browsers for every pull request, using Sauce Labs.

Polyfills link

Angular is built on the latest standards of the web platform. Targeting such a wide range of browsers is challenging because they do not support all features of modern browsers. You compensate by loading polyfill scripts ("polyfills") for the browsers that you must support. See instructions on how to include polyfills into your project below.

The suggested polyfills are the ones that run full Angular applications. You might need additional polyfills to support features not covered by this list.

Polyfills cannot magically transform an old, slow browser into a modern, fast one.

Enabling polyfills with CLI projects link

The Angular CLI provides support for polyfills. If you are not using the CLI to create your projects, see Polyfill instructions for non-CLI users.

The polyfills options of the browser and test builder can be a full path for a file (Example: src/polyfills.ts ) or, relative to the current workspace or module specifier (Example: zone.js ).

If you create a TypeScript file, make sure to include it in the files property of your tsconfig file.

  "extends": "./tsconfig.json",
  "compilerOptions": {
  "files": [

Polyfills for non-CLI users link

If you are not using the CLI, add your polyfill scripts directly to the host web page ( index.html ).

For example:

      <!-- pre-zone polyfills -->
<script src="node_modules/core-js/client/shim.min.js"></script>
   * you can configure some zone flags which can disable zone interception for some
   * asynchronous activities to improve startup performance - use these options only
   * if you know what you are doing as it could result in hard to trace down bugs.
  // __Zone_disable_requestAnimationFrame = true; // disable patch requestAnimationFrame
  // __Zone_disable_on_property = true; // disable patch onProperty such as onclick
  // __zone_symbol__UNPATCHED_EVENTS = ['scroll', 'mousemove']; // disable patch specified eventNames
   * in Edge developer tools, the addEventListener will also be wrapped by zone.js
   * with the following flag, it will bypass `zone.js` patch for Edge.
  // __Zone_enable_cross_context_check = true;
<!-- zone.js required by Angular -->
<script src="node_modules/zone.js/bundles/zone.umd.js"></script>
<!-- application polyfills -->
Last reviewed on Fri Nov 04 2022
Read article
Angular - Building and serving Angular apps

Building and serving Angular apps link

This page discusses build-specific configuration options for Angular projects.

Configuring application environments link

You can define different named build configurations for your project, such as staging and production , with different defaults.

Each named configuration can have defaults for any of the options that apply to the various builder targets, such as build , serve , and test . The Angular CLI build , serve , and test commands can then replace files with appropriate versions for your intended target environment.

Configure environment-specific defaults link

A project's src/environments/ folder contains the base configuration file, environment.ts , which provides a default environment. You can add override defaults for additional environments, such as production and staging, in target-specific configuration files.

For example:


The base file environment.ts , contains the default environment settings. For example:

      export const environment = {
  production: false

The build command uses this as the build target when no environment is specified. You can add further variables, either as additional properties on the environment object, or as separate objects. For example, the following adds a default for a variable to the default environment:

      export const environment = {
  production: false,
  apiUrl: 'http://my-api-url'

You can add target-specific configuration files, such as environment.prod.ts . The following content sets default values for the production build target:

      export const environment = {
  production: true,
  apiUrl: 'http://my-prod-url'

Using environment-specific variables in your app link

The following application structure configures build targets for production and staging environments:


To use the environment configurations you have defined, your components must import the original environments file:

      import { environment } from './../environments/environment';

This ensures that the build and serve commands can find the configurations for specific build targets.

The following code in the component file ( app.component.ts ) uses an environment variable defined in the configuration files.

      import { Component } from '@angular/core';
import { environment } from './../environments/environment';

  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
export class AppComponent {
  constructor() {
    console.log(environment.production); // Logs false for default environment
  title = 'app works!';

Configure target-specific file replacements link

The main CLI configuration file, angular.json , contains a fileReplacements section in the configuration for each build target, which lets you replace any file in the TypeScript program with a target-specific version of that file. This is useful for including target-specific code or variables in a build that targets a specific environment, such as production or staging.

By default no files are replaced. You can add file replacements for specific build targets. For example:

      "configurations": {
  "production": {
    "fileReplacements": [
        "replace": "src/environments/environment.ts",
        "with": "src/environments/environment.prod.ts"

This means that when you build your production configuration with ng build --configuration production , the src/environments/environment.ts file is replaced with the target-specific version of the file, src/environments/environment.prod.ts .

You can add additional configurations as required. To add a staging environment, create a copy of src/environments/environment.ts called src/environments/environment.staging.ts , then add a staging configuration to angular.json :

      "configurations": {
  "production": {  },
  "staging": {
    "fileReplacements": [
        "replace": "src/environments/environment.ts",
        "with": "src/environments/environment.staging.ts"

You can add more configuration options to this target environment as well. Any option that your build supports can be overridden in a build target configuration.

To build using the staging configuration, run the following command:

      ng build --configuration=staging

You can also configure the serve command to use the targeted build configuration if you add it to the "serve:configurations" section of angular.json :

      "serve": {
  "builder": "@angular-devkit/build-angular:dev-server",
  "options": {
    "browserTarget": "your-project-name:build"
  "configurations": {
    "production": {
      "browserTarget": "your-project-name:build:production"
    "staging": {
      "browserTarget": "your-project-name:build:staging"

Configuring size budgets link

As applications grow in functionality, they also grow in size. The CLI lets you set size thresholds in your configuration to ensure that parts of your application stay within size boundaries that you define.

Define your size boundaries in the CLI configuration file, angular.json , in a budgets section for each configured environment.

  "configurations": {
    "production": {
      "budgets": []

You can specify size budgets for the entire app, and for particular parts. Each budget entry configures a budget of a given type. Specify size values in the following formats:

Size value Details
123 or 123b Size in bytes.
123kb Size in kilobytes.
123mb Size in megabytes.
12% Percentage of size relative to baseline. (Not valid for baseline values.)

When you configure a budget, the build system warns or reports an error when a given part of the application reaches or exceeds a boundary size that you set.

Each budget entry is a JSON object with the following properties:

Property Value
type The type of budget. One of:
Value Details
bundle The size of a specific bundle.
initial The size of JavaScript needed for bootstrapping the application. Defaults to warning at 500kb and erroring at 1mb.
allScript The size of all scripts.
all The size of the entire application.
anyComponentStyle This size of any one component stylesheet. Defaults to warning at 2kb and erroring at 4kb.
anyScript The size of any one script.
any The size of any file.
name The name of the bundle (for type=bundle ).
baseline The baseline size for comparison.
maximumWarning The maximum threshold for warning relative to the baseline.
maximumError The maximum threshold for error relative to the baseline.
minimumWarning The minimum threshold for warning relative to the baseline.
minimumError The minimum threshold for error relative to the baseline.
warning The threshold for warning relative to the baseline (min & max).
error The threshold for error relative to the baseline (min & max).

Configuring CommonJS dependencies link

It is recommended that you avoid depending on CommonJS modules in your Angular applications. Depending on CommonJS modules can prevent bundlers and minifiers from optimizing your application, which results in larger bundle sizes. Instead, it is recommended that you use ECMAScript modules in your entire application. For more information, see How CommonJS is making your bundles larger.

The Angular CLI outputs warnings if it detects that your browser application depends on CommonJS modules. To disable these warnings, add the CommonJS module name to allowedCommonJsDependencies option in the build options located in angular.json file.

      "build": {
  "builder": "@angular-devkit/build-angular:browser",
  "options": {
     "allowedCommonJsDependencies": [

Configuring browser compatibility link

The Angular CLI uses Browserslist to ensure compatibility with different browser versions. Autoprefixer is used for CSS vendor prefixing and @babel/preset-env for JavaScript syntax transformations.

Internally, the Angular CLI uses the below browserslist configuration which matches the browsers that are supported by Angular.

      last 1 Chrome version
last 1 Firefox version
last 2 Edge major versions
last 2 Safari major versions
last 2 iOS major versions
Firefox ESR

To override the internal configuration, add a new file named .browserslistrc , to the project directory, that specifies the browsers you want to support:

      last 1 Chrome version
last 1 Firefox version

See the browserslist repository for more examples of how to target specific browsers and versions.

Use browsersl.ist to display compatible browsers for a browserslist query.

Proxying to a backend server link

Use the proxying support in the webpack development server to divert certain URLs to a backend server, by passing a file to the --proxy-config build option. For example, to divert all calls for http://localhost:4200/api to a server running on http://localhost:3000/api , take the following steps.

  1. Create a file proxy.conf.json in your project's src/ folder.

  2. Add the following content to the new proxy file:

      "/api": {
        "target": "http://localhost:3000",
        "secure": false
  3. In the CLI configuration file, angular.json , add the proxyConfig option to the serve target:

    "architect": {
      "serve": {
        "builder": "@angular-devkit/build-angular:dev-server",
        "options": {
          "browserTarget": "your-application-name:build",
          "proxyConfig": "src/proxy.conf.json"
  4. To run the development server with this proxy configuration, call ng serve .

Edit the proxy configuration file to add configuration options; following are some examples. For a description of all options, see webpack DevServer documentation.

If you edit the proxy configuration file, you must relaunch the ng serve process to make your changes effective.

Rewrite the URL path link

The pathRewrite proxy configuration option lets you rewrite the URL path at run time. For example, specify the following pathRewrite value to the proxy configuration to remove "api" from the end of a path.

  "/api": {
    "target": "http://localhost:3000",
    "secure": false,
    "pathRewrite": {
      "^/api": ""

If you need to access a backend that is not on localhost , set the changeOrigin option as well. For example:

  "/api": {
    "target": "http://npmjs.org",
    "secure": false,
    "pathRewrite": {
      "^/api": ""
    "changeOrigin": true

To help determine whether your proxy is working as intended, set the logLevel option. For example:

  "/api": {
    "target": "http://localhost:3000",
    "secure": false,
    "pathRewrite": {
      "^/api": ""
    "logLevel": "debug"

Proxy log levels are info (the default), debug , warn , error , and silent .

Proxy multiple entries link

You can proxy multiple entries to the same target by defining the configuration in JavaScript.

Set the proxy configuration file to proxy.conf.js (instead of proxy.conf.json ), and specify configuration files as in the following example.

      const PROXY_CONFIG = [
        context: [
        target: "http://localhost:3000",
        secure: false

module.exports = PROXY_CONFIG;

In the CLI configuration file, angular.json , point to the JavaScript proxy configuration file:

"architect": {
  "serve": {
    "builder": "@angular-devkit/build-angular:dev-server",
    "options": {
      "browserTarget": "your-application-name:build",
      "proxyConfig": "src/proxy.conf.js"


Bypass the proxy link

If you need to optionally bypass the proxy, or dynamically change the request before it's sent, add the bypass option, as shown in this JavaScript example.

      const PROXY_CONFIG = {
    "/api/proxy": {
        "target": "http://localhost:3000",
        "secure": false,
        "bypass": function (req, res, proxyOptions) {
            if (req.headers.accept.indexOf("html") !== -1) {
                console.log("Skipping proxy for browser request.");
                return "/index.html";
            req.headers["X-Custom-Header"] = "yes";

module.exports = PROXY_CONFIG;

Using corporate proxy link

If you work behind a corporate proxy, the backend cannot directly proxy calls to any URL outside your local network. In this case, you can configure the backend proxy to redirect calls through your corporate proxy using an agent:

      npm install --save-dev https-proxy-agent

When you define an environment variable http_proxy or HTTP_PROXY , an agent is automatically added to pass calls through your corporate proxy when running npm start .

Use the following content in the JavaScript configuration file.

      var HttpsProxyAgent = require('https-proxy-agent');
var proxyConfig = [{
  context: '/api',
  target: 'http://your-remote-server.com:3000',
  secure: false

function setupForCorporateProxy(proxyConfig) {
  var proxyServer = process.env.http_proxy || process.env.HTTP_PROXY;
  if (proxyServer) {
    var agent = new HttpsProxyAgent(proxyServer);
    console.log('Using corporate proxy server: ' + proxyServer);
    proxyConfig.forEach(function(entry) {
      entry.agent = agent;
  return proxyConfig;

module.exports = setupForCorporateProxy(proxyConfig);
Last reviewed on Mon Oct 24 2022
Read article
Angular - Built-in directives

Built-in directives link

Directives are classes that add additional behavior to elements in your Angular applications. Use Angular's built-in directives to manage forms, lists, styles, and what users see.

See the live example / download example for a working example containing the code snippets in this guide.

The different types of Angular directives are as follows:

Directive Types Details
Components Used with a template. This type of directive is the most common directive type.
Attribute directives Change the appearance or behavior of an element, component, or another directive.
Structural directives Change the DOM layout by adding and removing DOM elements.

This guide covers built-in attribute directives and structural directives.

Built-in attribute directives link

Attribute directives listen to and modify the behavior of other HTML elements, attributes, properties, and components.

Many NgModules such as the RouterModule and the FormsModule define their own attribute directives. The most common attribute directives are as follows:

Common directives Details
NgClass Adds and removes a set of CSS classes.
NgStyle Adds and removes a set of HTML styles.
NgModel Adds two-way data binding to an HTML form element.

Built-in directives use only public APIs. They do not have special access to any private APIs that other directives can't access.

Adding and removing classes with NgClass link

Add or remove multiple CSS classes simultaneously with ngClass .

To add or remove a single class, use class binding rather than NgClass .

Using NgClass with an expression link

On the element you'd like to style, add [ngClass] and set it equal to an expression. In this case, isSpecial is a boolean set to true in app.component.ts . Because isSpecial is true, ngClass applies the class of special to the <div> .

      <!-- toggle the "special" class on/off with a property -->
<div [ngClass]="isSpecial ? 'special' : ''">This div is special</div>

Using NgClass with a method link

  1. To use NgClass with a method, add the method to the component class. In the following example, setCurrentClasses() sets the property currentClasses with an object that adds or removes three classes based on the true or false state of three other component properties.

    Each key of the object is a CSS class name. If a key is true , ngClass adds the class. If a key is false , ngClass removes the class.

          currentClasses: Record<string, boolean> = {};
    /* . . . */
    setCurrentClasses() {
      // CSS classes: added/removed per current state of component properties
      this.currentClasses =  {
        saveable: this.canSave,
        modified: !this.isUnchanged,
        special:  this.isSpecial
  2. In the template, add the ngClass property binding to currentClasses to set the element's classes:

          <div [ngClass]="currentClasses">This div is initially saveable, unchanged, and special.</div>

For this use case, Angular applies the classes on initialization and in case of changes. The full example calls setCurrentClasses() initially with ngOnInit() and when the dependent properties change through a button click. These steps are not necessary to implement ngClass . For more information, see the live example / download example app.component.ts and app.component.html .

Setting inline styles with NgStyle link

Use NgStyle to set multiple inline styles simultaneously, based on the state of the component.

  1. To use NgStyle , add a method to the component class.

    In the following example, setCurrentStyles() sets the property currentStyles with an object that defines three styles, based on the state of three other component properties.

          currentStyles: Record<string, string> = {};
    /* . . . */
    setCurrentStyles() {
      // CSS styles: set per current state of component properties
      this.currentStyles = {
        'font-style':  this.canSave      ? 'italic' : 'normal',
        'font-weight': !this.isUnchanged ? 'bold'   : 'normal',
        'font-size':   this.isSpecial    ? '24px'   : '12px'
  2. To set the element's styles, add an ngStyle property binding to currentStyles .

          <div [ngStyle]="currentStyles">
      This div is initially italic, normal weight, and extra large (24px).

For this use case, Angular applies the styles upon initialization and in case of changes. To do this, the full example calls setCurrentStyles() initially with ngOnInit() and when the dependent properties change through a button click. However, these steps are not necessary to implement ngStyle on its own. See the live example / download example app.component.ts and app.component.html for this optional implementation.

Displaying and updating properties with ngModel link

Use the NgModel directive to display a data property and update that property when the user makes changes.

  1. Import FormsModule and add it to the NgModule's imports list.

    src/app/app.module.ts (FormsModule import)
          import { FormsModule } from '@angular/forms'; // <--- JavaScript import from Angular
    /* . . . */
      /* . . . */
      imports: [
        FormsModule // <--- import into the NgModule
      /* . . . */
    export class AppModule { }
  2. Add an [(ngModel)] binding on an HTML <form> element and set it equal to the property, here name .

    src/app/app.component.html (NgModel example)
          <label for="example-ngModel">[(ngModel)]:</label>
    <input [(ngModel)]="currentItem.name" id="example-ngModel">

    This [(ngModel)] syntax can only set a data-bound property.

To customize your configuration, write the expanded form, which separates the property and event binding. Use property binding to set the property and event binding to respond to changes. The following example changes the <input> value to uppercase:

      <input [ngModel]="currentItem.name" (ngModelChange)="setUppercaseName($event)" id="example-uppercase">

Here are all variations in action, including the uppercase version:

NgModel and value accessors link

The NgModel directive works for an element supported by a ControlValueAccessor. Angular provides value accessors for all of the basic HTML form elements. For more information, see Forms.

To apply [(ngModel)] to a non-form built-in element or a third-party custom component, you have to write a value accessor. For more information, see the API documentation on DefaultValueAccessor.

When you write an Angular component, you don't need a value accessor or NgModel if you name the value and event properties according to Angular's two-way binding syntax.

Built-in structural directives link

Structural directives are responsible for HTML layout. They shape or reshape the DOM's structure, typically by adding, removing, and manipulating the host elements to which they are attached.

This section introduces the most common built-in structural directives:

Common built-in structural directives Details
NgIf Conditionally creates or disposes of subviews from the template.
NgFor Repeat a node for each item in a list.
NgSwitch A set of directives that switch among alternative views.

For more information, see Structural Directives.

Adding or removing an element with NgIf link

Add or remove an element by applying an NgIf directive to a host element.

When NgIf is false , Angular removes an element and its descendants from the DOM. Angular then disposes of their components, which frees up memory and resources.

To add or remove an element, bind *ngIf to a condition expression such as isActive in the following example.

      <app-item-detail *ngIf="isActive" [item]="item"></app-item-detail>

When the isActive expression returns a truthy value, NgIf adds the ItemDetailComponent to the DOM. When the expression is falsy, NgIf removes the ItemDetailComponent from the DOM and disposes of the component and all of its subcomponents.

For more information on NgIf and NgIfElse , see the NgIf API documentation.

Guarding against null link

By default, NgIf prevents display of an element bound to a null value.

To use NgIf to guard a <div> , add *ngIf="yourProperty" to the <div> . In the following example, the currentCustomer name appears because there is a currentCustomer .

      <div *ngIf="currentCustomer">Hello, {{currentCustomer.name}}</div>

However, if the property is null , Angular does not display the <div> . In this example, Angular does not display the nullCustomer because it is null .

      <div *ngIf="nullCustomer">Hello, <span>{{nullCustomer}}</span></div>

Listing items with NgFor link

Use the NgFor directive to present a list of items.

  1. Define a block of HTML that determines how Angular renders a single item.
  2. To list your items, assign the shorthand let item of items to *ngFor .
      <div *ngFor="let item of items">{{item.name}}</div>

The string "let item of items" instructs Angular to do the following:

  • Store each item in the items array in the local item looping variable
  • Make each item available to the templated HTML for each iteration
  • Translate "let item of items" into an <ng-template> around the host element
  • Repeat the <ng-template> for each item in the list

For more information see the Structural directive shorthand section of Structural directives.

Repeating a component view link

To repeat a component element, apply *ngFor to the selector. In the following example, the selector is <app-item-detail> .

      <app-item-detail *ngFor="let item of items" [item]="item"></app-item-detail>

Reference a template input variable, such as item , in the following locations:

  • Within the ngFor host element
  • Within the host element descendants to access the item's properties

The following example references item first in an interpolation and then passes in a binding to the item property of the <app-item-detail> component.

      <div *ngFor="let item of items">{{item.name}}</div>
<!-- . . . -->
<app-item-detail *ngFor="let item of items" [item]="item"></app-item-detail>

For more information about template input variables, see Structural directive shorthand.

Getting the index of *ngFor link

Get the index of *ngFor in a template input variable and use it in the template.

In the *ngFor , add a semicolon and let i=index to the shorthand. The following example gets the index in a variable named i and displays it with the item name.

      <div *ngFor="let item of items; let i=index">{{i + 1}} - {{item.name}}</div>

The index property of the NgFor directive context returns the zero-based index of the item in each iteration.

Angular translates this instruction into an <ng-template> around the host element, then uses this template repeatedly to create a new set of elements and bindings for each item in the list. For more information about shorthand, see the Structural Directives guide.

Repeating elements when a condition is true link

To repeat a block of HTML when a particular condition is true, put the *ngIf on a container element that wraps an *ngFor element.

For more information see one structural directive per element.

Tracking items with *ngFor trackBy link

Reduce the number of calls your application makes to the server by tracking changes to an item list. With the *ngFor trackBy property, Angular can change and re-render only those items that have changed, rather than reloading the entire list of items.

  1. Add a method to the component that returns the value NgFor should track. In this example, the value to track is the item's id . If the browser has already rendered id , Angular keeps track of it and doesn't re-query the server for the same id .

          trackByItems(index: number, item: Item): number { return item.id; }
  2. In the shorthand expression, set trackBy to the trackByItems() method.

          <div *ngFor="let item of items; trackBy: trackByItems">
      ({{item.id}}) {{item.name}}

Change ids creates new items with new item.id s. In the following illustration of the trackBy effect, Reset items creates new items with the same item.id s.

  • With no trackBy , both buttons trigger complete DOM element replacement.
  • With trackBy , only changing the id triggers element replacement.

Hosting a directive without a DOM element link

The Angular <ng-container> is a grouping element that doesn't interfere with styles or layout because Angular doesn't put it in the DOM.

Use <ng-container> when there's no single element to host the directive.

Here's a conditional paragraph using <ng-container> .

src/app/app.component.html (ngif-ngcontainer)
  I turned the corner
  <ng-container *ngIf="hero">
    and saw {{hero.name}}. I waved
  and continued on my way.
  1. Import the ngModel directive from FormsModule .

  2. Add FormsModule to the imports section of the relevant Angular module.

  3. To conditionally exclude an <option> , wrap the <option> in an <ng-container> .

    src/app/app.component.html (select-ngcontainer)
      Pick your favorite hero
      (<label><input type="checkbox" checked (change)="showSad = !showSad">show sad</label>)
    <select [(ngModel)]="hero">
      <ng-container *ngFor="let h of heroes">
        <ng-container *ngIf="showSad || h.emotion !== 'sad'">
          <option [ngValue]="h">{{h.name}} ({{h.emotion}})</option>

Switching cases with NgSwitch link

Like the JavaScript switch statement, NgSwitch displays one element from among several possible elements, based on a switch condition. Angular puts only the selected element into the DOM.

NgSwitch is a set of three directives:

NgSwitch directives Details
NgSwitch An attribute directive that changes the behavior of its companion directives.
NgSwitchCase Structural directive that adds its element to the DOM when its bound value equals the switch value and removes its bound value when it doesn't equal the switch value.
NgSwitchDefault Structural directive that adds its element to the DOM when there is no selected NgSwitchCase .
  1. On an element, such as a <div> , add [ngSwitch] bound to an expression that returns the switch value, such as feature . Though the feature value in this example is a string, the switch value can be of any type.

  2. Bind to *ngSwitchCase and *ngSwitchDefault on the elements for the cases.

          <div [ngSwitch]="currentItem.feature">
      <app-stout-item    *ngSwitchCase="'stout'"    [item]="currentItem"></app-stout-item>
      <app-device-item   *ngSwitchCase="'slim'"     [item]="currentItem"></app-device-item>
      <app-lost-item     *ngSwitchCase="'vintage'"  [item]="currentItem"></app-lost-item>
      <app-best-item     *ngSwitchCase="'bright'"   [item]="currentItem"></app-best-item>
    <!-- . . . -->
      <app-unknown-item  *ngSwitchDefault           [item]="currentItem"></app-unknown-item>
  3. In the parent component, define currentItem , to use it in the [ngSwitch] expression.

          currentItem!: Item;
  4. In each child component, add an item input property which is bound to the currentItem of the parent component. The following two snippets show the parent component and one of the child components. The other child components are identical to StoutItemComponent .

    In each child component, here StoutItemComponent
          export class StoutItemComponent {
      @Input() item!: Item;

Switch directives also work with built-in HTML elements and web components. For example, you could replace the <app-best-item> switch case with a <div> as follows.

      <div *ngSwitchCase="'bright'"> Are you as bright as {{currentItem.name}}?</div>

What's next link

For information on how to build your own custom directives, see Attribute Directives and Structural Directives.

Last reviewed on Mon Feb 28 2022
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Angular - Skipping component subtrees

Skipping component subtrees link

JavaScript, by default, uses mutable data structures that you can reference from multiple different components. Angular runs change detection over your entire component tree to make sure that the most up-to-date state of your data structures is reflected in the DOM.

Change detection is sufficiently fast for most applications. However, when an application has an especially large component tree, running change detection across the whole application can cause performance issues. You can address this by configuring change detection to only run on a subset of the component tree.

If you are confident that a part of the application is not affected by a state change, you can use OnPush to skip change detection in an entire component subtree.

Using OnPush link

OnPush change detection instructs Angular to run change detection for a component subtree only when:

  • The root component of the subtree receives new inputs as the result of a template binding. Angular compares the current and past value of the input with ==
  • Angular handles an event (for example using event binding, output binding, or @HostListener ) in the subtree's root component or any of its children whether they are using OnPush change detection or not.

You can set the change detection strategy of a component to OnPush in the @Component decorator:

      import { ChangeDetectionStrategy, Component } from '@angular/core';
  changeDetection: ChangeDetectionStrategy.OnPush,
export class MyComponent {}

Common change detection scenarios link

This section examines several common change detection scenarios to illustrate Angular's behavior.

An event is handled by a component with default change detection link

If Angular handles an event within a component without OnPush strategy, the framework executes change detection on the entire component tree. Angular will skip descendant component subtrees with roots using OnPush , which have not received new inputs.

As an example, if we set the change detection strategy of MainComponent to OnPush and the user interacts with a component outside the subtree with root MainComponent , Angular will check all the green components from the diagram below ( AppComponent , HeaderComponent , SearchComponent , ButtonComponent ) unless MainComponent receives new inputs:

An event is handled by a component with OnPush link

If Angular handles an event within a component with OnPush strategy, the framework will execute change detection within the entire component tree. Angular will ignore component subtrees with roots using OnPush, which have not received new inputs and are outside the component which handled the event.

As an example, if Angular handles an event within MainComponent , the framework will run change detection in the entire component tree. Angular will ignore the subtree with root LoginComponent because it has OnPush and the event happened outside of its scope.

An event is handled by a descendant of a component with OnPush link

If Angular handles an event in a component with OnPush, the framework will execute change detection in the entire component tree, including the component’s ancestors.

As an example, in the diagram below, Angular handles an event in LoginComponent which uses OnPush. Angular will invoke change detection in the entire component subtree including MainComponent ( LoginComponent ’s parent), even though MainComponent has OnPush as well. Angular checks MainComponent as well because LoginComponent is part of its view.

New inputs to component with OnPush link

Angular will run change detection within a child component with OnPush setting an input property as result of a template binding.

For example, in the diagram below, AppComponent passes a new input to MainComponent , which has OnPush . Angular will run change detection in MainComponent but will not run change detection in LoginComponent , which also has OnPush , unless it receives new inputs as well.

Edge cases link

  • Modifying input properties in TypeScript code . When you use an API like @ViewChild or @ContentChild to get a reference to a component in TypeScript and manually modify an @Input property, Angular will not automatically run change detection for OnPush components. If you need Angular to run change detection, you can inject ChangeDetectorRef in your component and call changeDetectorRef.markForCheck() to tell Angular to schedule a change detection.
  • Modifying object references . In case an input receives a mutable object as value and you modify the object but preserve the reference, Angular will not invoke change detection. That’s the expected behavior because the previous and the current value of the input point to the same reference.
Last reviewed on Wed May 04 2022
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Angular - Slow computations

Slow computations link

On every change detection cycle, Angular synchronously:

  • Evaluates all template expressions in all components, unless specified otherwise, based on that each component's detection strategy
  • Executes the ngDoCheck , ngAfterContentChecked , ngAfterViewChecked , and ngOnChanges lifecycle hooks. A single slow computation within a template or a lifecycle hook can slow down the entire change detection process because Angular runs the computations sequentially.

Identifying slow computations link

You can identify heavy computations with Angular DevTools’ profiler. In the performance timeline, click a bar to preview a particular change detection cycle. This displays a bar chart, which shows how long the framework spent in change detection for each component. When you click a component, you can preview how long Angular spent evaluating its template and lifecycle hooks.

For example, in the preceding screenshot, the second recorded change detection cycle is selected. Angular spent over 573 ms on this cycle, with the most time spent in the EmployeeListComponent . In the details panel, you can see that Angular spent over 297 ms evaluating the template of the EmployeeListComponent .

Optimizing slow computations link

Here are several techniques to remove slow computations:

  • Optimizing the underlying algorithm . This is the recommended approach. If you can speed up the algorithm that is causing the problem, you can speed up the entire change detection mechanism.
  • Caching using pure pipes . You can move the heavy computation to a pure pipe. Angular reevaluates a pure pipe only if it detects that its inputs have changed, compared to the previous time Angular called it.
  • Using memoization . Memoization is a similar technique to pure pipes, with the difference that pure pipes preserve only the last result from the computation where memoization could store multiple results.
  • Avoid repaints/reflows in lifecycle hooks . Certain operations cause the browser to either synchronously recalculate the layout of the page or re-render it. Since reflows and repaints are generally slow, you want to avoid performing them in every change detection cycle.

Pure pipes and memoization have different trade-offs. Pure pipes are an Angular built-in concept compared to memoization, which is a general software engineering practice for caching function results. The memory overhead of memoization could be significant if you invoke the heavy computation frequently with different arguments.

Last reviewed on Wed May 04 2022
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