Interpolation & Property Binding in Angular 4

In this article, we will discuss about interpolation and property binding in Angular 4. Both are used to bind component class properties to view template.


Let’s take simple example of both

  • We will bind imagePath property of the component class to <img>element src property using interpolation as shown below.
    <img src='{{imagePath}}’/>
  • We can achieve the same using Property binding also. Check bellow example of property biding. <img [src]=’imagePath’/>
  • Here notice the <img>element src property is in a pair of square brackets, and the component class property is in quotes.

Interpolation and Property binding will drive value in one direction, from a component’s data property into target element property.

Difference between Interpolation and Property binding

  • Where to use Interpolation?
    • In some cases like when we need to concatenate strings we have to use interpolation instead of property binding.
    • Check this example.
      <img src=’{{imagePath}}’ />
  • Where to use Property binding?
    • When setting an element property to a non-string data value, you must use property binding.
    • In the following example, we are disabling a button by binding to the boolean property isDisabled.
      <button [disabled]=’isDisabled’>Click me</button>
    • If we use interpolation instead of property binding, the button is always disabled irrespective of isDisabled class property value.
      <button disabled='{{isDisabled}}’>Click me</button> (this doesn’t work).

 Points to keep in mind when using Property binding

  • Remember to enclose the property name with a pair of square brackets. If you omit the brackets, Angular treats the string as a constant and initializes the target property with that string.
    <span [innerHTML]=’pageHeader’></span>
  • With Property binding we enclose the element property name in square brackets
    <button [disabled]=’isDisabled’>Click me</button>
  • We can also use the alternate syntax with bind- prefix. This is known as canonical form
    <button bind-disabled=’isDisabled’>Click me</button>

 Provide security from malicious content 

  • From security standpoint, Angular data binding sanitizes malicious content before displaying it on the browser.
  • Both interpolation and property binding protects us from malicious content.
  • Check bellow example, we are using interpolation. Notice the malicious usage of <script> tag.
import { Component } from '@angular/core';

    selector: 'my-app',
    template: '<div>{{badHtml}}</div>'

export class AppComponent {
    badHtml: string = 'Hello <script>alert("Hacking");</script> World';
  • Angular interpolation sanitizes the malicious content and displays the following in the browser
    Hello <script>alert(“Hacking “);</script> World
  • Let’s change the code and use property binding.
    ‘<div [innerHtml]=”badHtml”></div>’
  • Property binding sanitizes the malicious content slightly differently and we get the following output : Hello alert(“Hacking”); World

So here the important point to keep in mind is both the techniques protect us from malicious content and render the content harmlessly.


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