30.2. One-Way Data Binding

Linking data from a component.ts file to the component.html file is an example of one-way binding. Changes made to the component.ts file are reflected in the view, but changes made in component.html have no influence on component.ts. Information flows in one direction only.

30.2.1. Displaying the Values of Variables

As we saw in Angular lessons 1 and 2, the syntax for binding a variable is to encase its name in double braces {{ }}. We used this in the exercises and studios to display headings, titles from our movie list, astronauts on our crew, etc.

Example

Inside the my-list.component.ts file:

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export class MyListComponent implements OnInit {
   listHeading: string = 'My Great List';
   luckyNumber: number = 42;

   constructor() { }

   ngOnInit() { }
}

Inside the my-list.component.html file:

<h3>{{listHeading}}</h3>
<p>{{luckyNumber}}<p>

Changing the value of luckyNumber in the MyListComponent class will change the text displayed by the p element. However, altering the HTML will NOT affect the value of luckyNumber in the class.

30.2.2. Binding to HTML Attributes

We also used one-way data binding to modify attributes within an HTML tag. For example, assume the image variable holds the URL for a photo we want to use on our web page.

Example

Inside the photos.component.ts file:

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export class PhotosComponent implements OnInit {
   image: string = 'https://www.launchcode.org/assets/icons/trophy-95e8cbe9bfda44123422302951deb1c92a237d39052669b8fbfafec00cb4f608.png';

   constructor() { }

   ngOnInit() { }
}

Inside the photos.component.html file:

<img src="{{image}}"/>

Within the <img> tag, we bind the image variable to the src attribute. Note that the braces and variable name must be inside quotes. When the code runs, the string stored in image is used to provide the required URL.

30.2.2.1. Update on the Binding Syntax

Angular allows us to use an alternate syntax whenever we use data-binding to modify an HTML attribute.

To avoid cluttering up our HTML tags with lots of double braces {{ }}, we simplify the notation as follows:

Examples

Double braces syntax:

<img src="{{ variableName }}"/>
<input name="{{ otherVariable }}" value="{{ thirdVariable }}"/>

Alternate syntax:

<img [src]="variableName"/>
<input [name]="otherVariable" [value]="thirdVariable"/>

Instead of {{ }}, place the HTML attribute in square brackets [ ] and set that equal to the variable name in quotes.

Note: Although the two methods accomplish exactly the same thing, the square brackets syntax is recommended as a best practice.

30.2.3. Check Your Understanding

Question

Which of the following is NOT a true statement about data-binding?

  1. Data-binding refers to the linking of component information to a view.
  2. In one-way binding, information flows in one direction only.
  3. In one-way binding, changing a component variable never updates the application view.
  4. In one-way binding, user input does not affect component data.

Question

Which of the following show proper data-binding syntax? Choose ALL that apply.

  1. <li>{{ variableName }}</li>
  2. <img [src]="variableName"/>
  3. <p>[variableName]</p>
  4. <button value="[variableName]">Go!</button>
  5. <input placeholder="{{ variableName }}"/>
  6. <button [name]="{{ variableName }}"/>