In an Angular template, a binding creates a live connection between a part of the UI created from a template (a DOM element, directive, or component) and the model (the component instance to which the template belongs). This connection can be used to synchronize the view with the model, to notify the model when an event or user action takes place in the view, or both. Angular's Change Detection algorithm is responsible for keeping the view and the model in sync.
Examples of binding include:
Bindings always have two parts: a target which will receive the bound value, and a template expression which produces a value from the model.
Template expressions are similar to JavaScript expressions. Many JavaScript expressions are legal template expressions, with the following exceptions.
You can't use JavaScript expressions that have or promote side effects, including:
=
, +=
, -=
, ...
)new
, typeof
, or instanceof
;
or ,
++
and --
Other notable differences from JavaScript syntax include:
|
and &
|
Interpolated expressions have a context—a particular part of the application to which the expression belongs. Typically, this context is the component instance.
In the following snippet, the expression recommended
and the expression itemImageUrl2
refer to properties of the AppComponent
.
<h4>{{recommended}}</h4> <img alt="item 2" [src]="itemImageUrl2">
An expression can also refer to properties of the template's context such as a template input variable or a template reference variable.
The following example uses a template input variable of customer
.
<ul> <li *ngFor="let customer of customers">{{customer.name}}</li> </ul>
This next example features a template reference variable, #customerInput
.
<label>Type something: <input #customerInput>{{customerInput.value}} </label>
Template expressions cannot refer to anything in the global namespace, except
undefined
. They can't refer towindow
ordocument
. Additionally, they can't callconsole.log()
orMath.max()
and are restricted to referencing members of the expression context.
The context against which an expression evaluates is the union of the template variables, the directive's context object—if it has one—and the component's members. If you reference a name that belongs to more than one of these namespaces, Angular applies the following precedence logic to determine the context:
To avoid variables shadowing variables in another context, keep variable names unique. In the following example, the AppComponent
template greets the customer
, Padma.
An ngFor
then lists each customer
in the customers
array.
@Component({ template: ` <div> <!-- Hello, Padma --> <h1>Hello, {{customer}}</h1> <ul> <!-- Ebony and Chiho in a list--> <li *ngFor="let customer of customers">{{ customer.value }}</li> </ul> </div> ` }) class AppComponent { customers = [{value: 'Ebony'}, {value: 'Chiho'}]; customer = 'Padma'; }
The customer
within the ngFor
is in the context of an <ng-template>
and so refers to the customer
in the customers
array, in this case Ebony and Chiho. This list does not feature Padma because customer
outside of the ngFor
is in a different context. Conversely, customer
in the <h1>
doesn't include Ebony or Chiho because the context for this customer
is the class and the class value for customer
is Padma.
When using a template expression, follow these best practices:
Use property names or method calls whenever possible. Keep application and business logic in the component, where it is accessible to develop and test.
Angular executes a template expression after every change detection cycle. Many asynchronous activities trigger change detection cycles, such as promise resolutions, HTTP results, timer events, key presses, and mouse moves.
An expression should finish quickly to keep the user experience as efficient as possible, especially on slower devices. Consider caching values when their computation requires greater resources.
According to Angular's unidirectional data flow model, a template expression should not change any application state other than the value of the target property. Reading a component value should not change some other displayed value. The view should be stable throughout a single rendering pass.
An idempotent expression is free of side effects and improves Angular's change detection performance. In Angular terms, an idempotent expression always returns exactly the same thing until one of its dependent values changes.
Dependent values should not change during a single turn of the event loop. If an idempotent expression returns a string or a number, it returns the same string or number if you call it twice consecutively. If the expression returns an object, including an array
, it returns the same object reference if you call it twice consecutively.
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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/guide/binding-overview