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Angular template


In AngularJS, templates are written with HTML that contains AngularJS-specific elements and attributes. ... These are the types of AngularJS elements and attributes you can use: Directive — An attribute or element that augments an existing DOM element or represents a reusable DOM component.


Template Syntax

The Angular application manages what the user sees and can do, achieving this through the interaction of a component class instance (the component) and its user-facing template.

You may be familiar with the component/template duality from your experience with model-view-controller (MVC) or model-view-viewmodel (MVVM). In Angular, the component plays the part of the controller/viewmodel, and the template represents the view.

This page is a comprehensive technical reference to the Angular template language. It explains basic principles of the template language and describes most of the syntax that you'll encounter elsewhere in the documentation.


HTML in templates

HTML is the language of the Angular template. Almost all HTML syntax is valid template syntax. The <script> element is a notable exception; it is forbidden, eliminating the risk of script injection attacks. In practice, <script> is ignored and a warning appears in the browser console. Some legal HTML doesn't make much sense in a template. The <html>, <body>, and <base> elements have no useful role. Pretty much everything else is fair game.

You can extend the HTML vocabulary of your templates with components and directives that appear as new elements and attributes. In the following sections, you'll learn how to get and set DOM (Document Object Model) values dynamically through data binding.


Expression guidelines

When using template expressions follow these guidelines:

  • Simplicity 
  • Quick execution
  • No visible side effects 

Simplicity

Although it's possible to write complex template expressions, it's a better practice to avoid them.

A property name or method call should be the norm, but an occasional Boolean negation, !, is OK. Otherwise, confine application and business logic to the component, where it is easier to develop and test.

Quick execution

Angular executes template expressions after every change detection cycle. Change detection cycles are triggered by many asynchronous activities such as promise resolutions, HTTP results, timer events, key presses and mouse moves.

Expressions should finish quickly or the user experience may drag, especially on slower devices. Consider caching values when their computation is expensive.

No visible side effects

A template expression should not change any application state other than the value of the target property.

This rule is essential to Angular's "unidirectional data flow" policy. You should never worry that reading a component value might change some other displayed value. The view should be stable throughout a single rendering pass.

An idempotent expression is ideal because it 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 when called twice in a row. If the expression returns an object, including an array, it returns the same object reference when called twice in a row.