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Property binding

Property binding in Angular helps you set values for properties of HTML elements or directives. With property binding, you can do things such as toggle button functionality, set paths programmatically, and share values between components.

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

Prerequisites

To get the most out of property binding, you should be familiar with the following:

Understanding the flow of data

Property binding moves a value in one direction, from a component's property into a target element property.

For more information on listening for events, see Event binding.

To read a target element property or call one of its methods, see the API reference for ViewChild and ContentChild.

Binding to a property

To bind to an element's property, enclose it in square brackets, [], which identifies the property as a target property. A target property is the DOM property to which you want to assign a value. For example, the target property in the following code is the image element's src property.

<img [src]="itemImageUrl">

In most cases, the target name is the name of a property, even when it appears to be the name of an attribute. In this example, src is the name of the <img> element property.

The brackets, [], cause Angular to evaluate the right-hand side of the assignment as a dynamic expression. Without the brackets, Angular treats the the right-hand side as a string literal and sets the property to that static value.

<app-item-detail childItem="parentItem"></app-item-detail>

Omitting the brackets renders the string parentItem, not the value of parentItem.

Setting an element property to a component property value

To bind the src property of an <img> element to a component's property, place the target, src, in square brackets followed by an equal sign and then the property. The property here is itemImageUrl.

<img [src]="itemImageUrl">

Declare the itemImageUrl property in the class, in this case AppComponent.

itemImageUrl = '../assets/phone.png';

colspan and colSpan

A common point of confusion is between the attribute, colspan, and the property, colSpan. Notice that these two names differ by only a single letter.

If you wrote something like this:

<tr><td colspan="{{1 + 1}}">Three-Four</td></tr>

You'd get this error:

Template parse errors:
  Can't bind to 'colspan' since it isn't a known native property

As the message says, the <td> element does not have a colspan property. This is true because colspan is an attribute—colSpan, with a capital S, is the corresponding property. Interpolation and property binding can set only properties, not attributes.

Instead, you'd use property binding and write it like this:

<!-- Notice the colSpan property is camel case -->
<tr><td [colSpan]="1 + 1">Three-Four</td></tr>

Another example is disabling a button when the component says that it isUnchanged:

<!-- Bind button disabled state to `isUnchanged` property -->
<button [disabled]="isUnchanged">Disabled Button</button>

Another is setting a property of a directive:

<p [ngClass]="classes">[ngClass] binding to the classes property making this blue</p>

Yet another is setting the model property of a custom component—a great way for parent and child components to communicate:

<app-item-detail [childItem]="parentItem"></app-item-detail>

Toggling button functionality

To disable a button's functionality depending on a Boolean value, bind the DOM disabled property to a property in the class that is true or false.

<!-- Bind button disabled state to `isUnchanged` property -->
<button [disabled]="isUnchanged">Disabled Button</button>

Because the value of the property isUnchanged is true in the AppComponent, Angular disables the button.

isUnchanged = true;

Setting a directive property

To set a property of a directive, place the directive within square brackets , such as [ngClass], followed by an equal sign and the property. Here, the property is classes.

<p [ngClass]="classes">[ngClass] binding to the classes property making this blue</p>

To use the property, you must declare it in the class, which in this example is AppComponent. The value of classes is special.

classes = 'special';

Angular applies the class special to the <p> element so that you can use special to apply CSS styles.

Bind values between components

To set the model property of a custom component, place the target, here childItem, between square brackets [] followed by an equal sign and the property. Here, the property is parentItem.

<app-item-detail [childItem]="parentItem"></app-item-detail>

To use the target and the property, you must declare them in their respective classes.

Declare the target of childItem in its component class, in this case ItemDetailComponent.

For example, the following code declares the target of childItem in its component class, in this case ItemDetailComponent.

Then, the code contains an @Input() decorator with the childItem property so data can flow into it.

@Input() childItem: string;

Next, the code declares the property of parentItem in its component class, in this case AppComponent. In this example the type of childItem is string, so parentItem needs to be a string. Here, parentItem has the string value of lamp.

parentItem = 'lamp';

With this configuration, the view of <app-item-detail> uses the value of lamp for childItem.

Property binding and security

Property binding can help keep content secure. For example, consider the following malicious content.

evilTitle = 'Template <script>alert("evil never sleeps")</script> Syntax';

The component template interpolates the content as follows:

<p><span>"{{evilTitle}}" is the <i>interpolated</i> evil title.</span></p>

The browser doesn't process the HTML and instead displays it raw, as follows.

"Template <script>alert("evil never sleeps")</script> Syntax" is the interpolated evil title.

Angular does not allow HTML with <script> tags, neither with interpolation nor property binding, which prevents the JavaScript from running.

In the following example, however, Angular sanitizes the values before displaying them.

<!--
 Angular generates a warning for the following line as it sanitizes them
 WARNING: sanitizing HTML stripped some content (see http://g.co/ng/security#xss).
-->
 <p>"<span [innerHTML]="evilTitle"></span>" is the <i>property bound</i> evil title.</p>

Interpolation handles the <script> tags differently than property binding, but both approaches render the content harmlessly. The following is the browser output of the sanitized evilTitle example.

"Template Syntax" is the property bound evil title.

Property binding and interpolation

Often interpolation and property binding can achieve the same results. The following binding pairs do the same thing.

<p><img src="{{itemImageUrl}}"> is the <i>interpolated</i> image.</p>
<p><img [src]="itemImageUrl"> is the <i>property bound</i> image.</p>

<p><span>"{{interpolationTitle}}" is the <i>interpolated</i> title.</span></p>
<p>"<span [innerHTML]="propertyTitle"></span>" is the <i>property bound</i> title.</p>

You can use either form when rendering data values as strings, though interpolation is preferable for readability. However, when setting an element property to a non-string data value, you must use property binding.

What's next

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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/guide/property-binding