We typically display data in Angular by binding controls in an HTML template to properties of an Angular component.
In this chapter, we'll create a component with a list of heroes. Each hero has a name. We'll display the list of hero names and conditionally show a message below the list.
The final UI looks like this:
Table Of Contents
- Showing component properties with interpolation
- Showing an array property with NgFor
- Conditional display with NgIf
The live example demonstrates all of the syntax and code snippets described in this chapter.
Showing component properties with interpolation
The easiest way to display a component property is to bind the property name through interpolation. With interpolation, we put the property name in the view template, enclosed in double curly braces: {{myHero}}.
Let's build a small illustrative example together.
Create a new project folder () and follow the steps in the QuickStart.
Alternatively, begin with a download of the QuickStart source.
Then modify the file by changing the template and the body of the component. When we're done, it should look like this:
app/app.component.ts
import { Component } from '@angular/core';
@Component({
selector: 'my-app',
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero}}</h2>
`
})
export class AppComponent {
title = 'Tour of Heroes';
myHero = 'Windstorm';
}
We added two properties to the formerly empty component: title and myHero.
Our revised template displays the two component properties using double curly brace interpolation:
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero}}</h2>
`
The template is a multi-line string within ECMAScript 2015 backticks (`). The backtick (`) — which is not the same character as a single quote (') — has many nice features. The feature we're exploiting here is the ability to compose the string over several lines, which makes for much more readable HTML.
Angular automatically pulls the value of the title and myHero properties from the component and inserts those values into the browser. Angular updates the display when these properties change.
More precisely, the redisplay occurs after some kind of asynchronous event related to the view such as a keystroke, a timer completion, or an async XHR response. We don't have those in this sample. But then the properties aren't changing on their own either. For the moment we must operate on faith.
Notice that we haven't called new to create an instance of the AppComponent class. Angular is creating an instance for us. How?
Notice the CSS selector in the @Component decorator that specifies an element named my-app. Remember back in QuickStart that we added the <my-app> element to the body of our index.html file:
index.html (body)
<body> <my-app>loading...</my-app> </body>
When we bootstrap with the AppComponent class (in ), Angular looks for a <my-app> in the index.html, finds it, instantiates an instance of AppComponent, and renders it inside the <my-app> tag.
Try running the app. It should display the title and hero name:
Let's review some of the choices we made and consider alternatives.
Template inline or template file?
We can store our component's template in one of two places. We can define it inline using the template property, as we do here. Or we can define the template in a separate HTML file and link to it in the component metadata using the @Component decorator's templateUrl property.
The choice between inline and separate HTML is a matter of taste, circumstances, and organization policy. Here we're using inline HTML because the template is small, and the demo is simpler without the additional HTML file.
In either style, the template data bindings have the same access to the component's properties.
Constructor or variable initialization?
We initialized our component properties using variable assignment. This is a wonderfully concise and compact technique.
Some folks prefer to declare the properties and initialize them within a constructor like this:
app/app-ctor.component.ts (class)
export class AppCtorComponent {
title: string;
myHero: string;
constructor() {
this.title = 'Tour of Heroes';
this.myHero = 'Windstorm';
}
}
That's fine too. The choice is a matter of taste and organization policy. We'll adopt the more terse "variable assignment" style in this chapter simply because there will be less code to read.
Showing an array property with *ngFor
We want to display a list of heroes. We begin by adding an array of hero names to the component and redefine myHero to be the first name in the array.
app/app.component.ts (class)
export class AppComponent {
title = 'Tour of Heroes';
heroes = ['Windstorm', 'Bombasto', 'Magneta', 'Tornado'];
myHero = this.heroes[0];
}
Now we use the Angular ngFor directive in the template to display each item in the heroes list.
app/app.component.ts (template)
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero}}</h2>
<p>Heroes:</p>
<ul>
<li *ngFor="let hero of heroes">
{{ hero }}
</li>
</ul>
`
Our presentation is the familiar HTML unordered list with <ul> and <li> tags. Let's focus on the <li> tag.
<li *ngFor="let hero of heroes">
{{ hero }}
</li>
We added a somewhat mysterious *ngFor to the <li> element. That's the Angular "repeater" directive. Its presence on the <li> tag marks that <li> element (and its children) as the "repeater template".
Don't forget the leading asterisk (*) in *ngFor. It is an essential part of the syntax. Learn more about this and ngFor in the Template Syntax chapter.
Notice the hero in the ngFor double-quoted instruction; it is an example of a template input variable.
Angular duplicates the <li> for each item in the list, setting the hero variable to the item (the hero) in the current iteration. Angular uses that variable as the context for the interpolation in the double curly braces.
We happened to give ngFor an array to display. In fact, ngFor can repeat items for any iterable object.
Now the heroes appear in an unordered list.
Creating a class for the data
We are defining our data directly inside our component. That's fine for a demo but certainly isn't a best practice. It's not even a good practice. Although we won't do anything about that in this chapter, we'll make a mental note to fix this down the road.
At the moment, we're binding to an array of strings. We do that occasionally in real applications, but most of the time we're binding to more specialized objects.
Let's turn our array of hero names into an array of Hero objects. For that we'll need a Hero class.
Create a new file in the app folder called with the following code:
app/hero.ts (excerpt)
export class Hero {
constructor(
public id: number,
public name: string) { }
}
We've defined a class with a constructor and two properties: id and name.
It might not look like we have properties, but we do. We're taking advantage of a TypeScript shortcut in our declaration of the constructor parameters.
Consider the first parameter:
app/hero.ts (id)
public id: number,
That brief syntax does a lot:
- Declares a constructor parameter and its type
- Declares a public property of the same name
- Initializes that property with the corresponding argument when we "new" an instance of the class
Using the Hero class
Let's make the heroes property in our component return an array of these Hero objects.
app/app.component.ts (heroes)
heroes = [ new Hero(1, 'Windstorm'), new Hero(13, 'Bombasto'), new Hero(15, 'Magneta'), new Hero(20, 'Tornado') ]; myHero = this.heroes[0];
We'll have to update the template. At the moment it displays the hero's id and name. Let's fix that so we display only the hero's name property.
app/app.component.ts (template)
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero.name}}</h2>
<p>Heroes:</p>
<ul>
<li *ngFor="let hero of heroes">
{{ hero.name }}
</li>
</ul>
`
Our display looks the same, but now we know much better what a hero really is.
Conditional display with NgIf
Sometimes an app needs to display a view or a portion of a view only under specific circumstances.
In our example, we'd like to display a message if we have a large number of heroes, say, more than 3.
The Angular ngIf directive inserts or removes an element based on a truthy/falsey condition. We can see it in action by adding the following paragraph at the bottom of the template:
app/app.component.ts (message)
<p *ngIf="heroes.length > 3">There are many heroes!</p>
Don't forget the leading asterisk (*) in *ngIf. It is an essential part of the syntax. Learn more about this and ngIf in the Template Syntax chapter.
The template expression inside the double quotes looks much like TypeScript, and it is much like TypeScript. When the component's list of heroes has more than 3 items, Angular adds the paragraph to the DOM and the message appears. If there are 3 or fewer items, Angular omits the paragraph, so no message appears.
Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That hardly matters here. But it would matter a great deal, from a performance perspective, if we were conditionally including or excluding a big chunk of HTML with many data bindings.
Try it out. Because the array has four items, the message should appear. Go back into and delete or comment out one of the elements from the hero array. The browser should refresh automatically and the message should disappear.
Summary
Now we know how to use:
- Interpolation with double curly braces to display a component property
- ngFor to display an array of items
- A TypeScript class to shape the model data for our component and display properties of that model
- ngIf to conditionally display a chunk of HTML based on a boolean expression
Here's our final code:
import { Component } from '@angular/core';
import { Hero } from './hero';
@Component({
selector: 'my-app',
template: `
<h1>{{title}}</h1>
<h2>My favorite hero is: {{myHero.name}}</h2>
<p>Heroes:</p>
<ul>
<li *ngFor="let hero of heroes">
{{ hero.name }}
</li>
</ul>
<p *ngIf="heroes.length > 3">There are many heroes!</p>
`
})
export class AppComponent {
title = 'Tour of Heroes';
heroes = [
new Hero(1, 'Windstorm'),
new Hero(13, 'Bombasto'),
new Hero(15, 'Magneta'),
new Hero(20, 'Tornado')
];
myHero = this.heroes[0];
}
export class Hero {
constructor(
public id: number,
public name: string) { }
}
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
@NgModule({
imports: [
BrowserModule
],
declarations: [
AppComponent
],
bootstrap: [ AppComponent ]
})
export class AppModule { }
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app.module';
platformBrowserDynamic().bootstrapModule(AppModule);
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