expression.ngdoc

@ngdoc overview
@name  Expressions
@sortOrder 270
@description

# Angular Expressions

Angular expressions are JavaScript-like code snippets that are usually placed in bindings such as
`{{ expression }}`.

For example, these are valid expressions in Angular:

  * `1+2`
  * `a+b`
  * `user.name`
  * `items[index]`


## Angular Expressions vs. JavaScript Expressions

Angular expressions are like JavaScript expressions with the following differences:

  * **Context:** JavaScript expressions are evaluated against the global `window`.
    In Angular, expressions are evaluated against a {@link ng.$rootScope.Scope `scope`} object.

  * **Forgiving:** In JavaScript, trying to evaluate undefined properties generates `ReferenceError`
    or `TypeError`. In Angular, expression evaluation is forgiving to `undefined` and `null`.

  * **No Control Flow Statements:** You cannot use the following in an Angular expression:
    conditionals, loops, or exceptions.
	
  * **No Function Declarations:** You cannot declare functions in an Angular expression,
    even inside `ng-init` directive.
	
  * **No RegExp Creation With Literal Notation:** You cannot create regular expressions 
    in an Angular expression.
	
  * **No Comma And Void Operators:** You cannot use `,` or `void` in an Angular expression.

  * **Filters:** You can use {@link guide/filter filters} within expressions to format data before
    displaying it.

If you want to run more complex JavaScript code, you should make it a controller method and call
the method from your view. If you want to `eval()` an Angular expression yourself, use the
{@link ng.$rootScope.Scope#$eval `$eval()`} method.

## Example
<example>
  <file name="index.html">
    <span>
      1+2={{1+2}}
    </span>
  </file>

  <file name="protractor.js" type="protractor">
    it('should calculate expression in binding', function() {
      expect(element(by.binding('1+2')).getText()).toEqual('1+2=3');
    });
  </file>
</example>

You can try evaluating different expressions here:

<example module="expressionExample">
  <file name="index.html">
    <div ng-controller="ExampleController" class="expressions">
      Expression:
      <input type='text' ng-model="expr" size="80"/>
      <button ng-click="addExp(expr)">Evaluate</button>
      <ul>
       <li ng-repeat="expr in exprs track by $index">
         [ <a href="" ng-click="removeExp($index)">X</a> ]
         <tt>{{expr}}</tt> => <span ng-bind="$parent.$eval(expr)"></span>
        </li>
      </ul>
    </div>
  </file>

  <file name="script.js">
    angular.module('expressionExample', [])
      .controller('ExampleController', ['$scope', function($scope) {
        var exprs = $scope.exprs = [];
        $scope.expr = '3*10|currency';
        $scope.addExp = function(expr) {
          exprs.push(expr);
        };

        $scope.removeExp = function(index) {
          exprs.splice(index, 1);
        };
      }]);
  </file>

  <file name="protractor.js" type="protractor">
    it('should allow user expression testing', function() {
      element(by.css('.expressions button')).click();
      var lis = element(by.css('.expressions ul')).all(by.repeater('expr in exprs'));
      expect(lis.count()).toBe(1);
      expect(lis.get(0).getText()).toEqual('[ X ] 3*10|currency => $30.00');
    });
  </file>
</example>


## Context

Angular does not use JavaScript's `eval()` to evaluate expressions. Instead Angular's
{@link ng.$parse $parse} service processes these expressions.

Angular expressions do not have access to global variables like `window`, `document` or `location`.
This restriction is intentional. It prevents accidental access to the global state – a common source of subtle bugs.

Instead use services like `$window` and `$location` in functions called from expressions. Such services
provide mockable access to globals.

<example module="expressionExample">
  <file name="index.html">
    <div class="example2" ng-controller="ExampleController">
      Name: <input ng-model="name" type="text"/>
      <button ng-click="greet()">Greet</button>
      <button ng-click="window.alert('Should not see me')">Won't greet</button>
    </div>
  </file>

  <file name="script.js">
    angular.module('expressionExample', [])
      .controller('ExampleController', ['$window', '$scope', function($window, $scope) {
        $scope.name = 'World';

        $scope.greet = function() {
          $window.alert('Hello ' + $scope.name);
        };
      }]);
  </file>

  <file name="protractor.js" type="protractor">
    it('should calculate expression in binding', function() {
      if (browser.params.browser == 'safari') {
        // Safari can't handle dialogs.
        return;
      }
      element(by.css('[ng-click="greet()"]')).click();

      var alertDialog = browser.switchTo().alert();

      expect(alertDialog.getText()).toEqual('Hello World');

      alertDialog.accept();
    });
  </file>
</example>

## Forgiving

Expression evaluation is forgiving to undefined and null. In JavaScript, evaluating `a.b.c` throws
an exception if `a` is not an object. While this makes sense for a general purpose language, the
expression evaluations are primarily used for data binding, which often look like this:

        {{a.b.c}}

It makes more sense to show nothing than to throw an exception if `a` is undefined (perhaps we are
waiting for the server response, and it will become defined soon). If expression evaluation wasn't
forgiving we'd have to write bindings that clutter the code, for example: `{{((a||{}).b||{}).c}}`

Similarly, invoking a function `a.b.c()` on `undefined` or `null` simply returns `undefined`.


## No Control Flow Statements

Apart from the ternary operator (`a ? b : c`), you cannot write a control flow statement in an
expression. The reason behind this is core to the Angular philosophy that application logic should
be in controllers, not the views. If you need a real conditional, loop, or to throw from a view
expression, delegate to a JavaScript method instead.

## No function declarations or RegExp creation with literal notation

You can't declare functions or create regular expressions from within AngularJS expressions. This is
to avoid complex model transformation logic inside templates. Such logic is better placed in a 
controller or in a dedicated filter where it can be tested properly.

## `$event`

Directives like {@link ng.directive:ngClick `ngClick`} and {@link ng.directive:ngFocus `ngFocus`}
expose a `$event` object within the scope of that expression. The object is an instance of a [jQuery
Event Object](http://api.jquery.com/category/events/event-object/) when jQuery is present or a
similar jqLite object.

<example module="eventExampleApp">
  <file name="index.html">
    <div ng-controller="EventController">
      <button ng-click="clickMe($event)">Event</button>
      <p><code>$event</code>: <pre> {{$event | json}}</pre></p>
      <p><code>clickEvent</code>: <pre>{{clickEvent | json}}</pre></p>
    </div>
  </file>

  <file name="script.js">
    angular.module('eventExampleApp', []).
      controller('EventController', ['$scope', function($scope) {
        /*
         * expose the event object to the scope
         */
        $scope.clickMe = function(clickEvent) {
          $scope.clickEvent = simpleKeys(clickEvent);
          console.log(clickEvent);
        };

        /*
         * return a copy of an object with only non-object keys
         * we need this to avoid circular references
         */
        function simpleKeys (original) {
          return Object.keys(original).reduce(function (obj, key) {
            obj[key] = typeof original[key] === 'object' ? '{ ... }' : original[key];
            return obj;
          }, {});
        }
      }]);
  </file>
</example>

Note in the example above how we can pass in `$event` to `clickMe`, but how it does not show up
in `{{$event}}`. This is because `$event` is outside the scope of that binding.


## One-time binding

An expression that starts with `::` is considered a one-time expression. One-time expressions
will stop recalculating once they are stable, which happens after the first digest if the expression
result is a non-undefined value (see value stabilization algorithm below).

<example module="oneTimeBidingExampleApp">
  <file name="index.html">
    <div ng-controller="EventController">
      <button ng-click="clickMe($event)">Click Me</button>
      <p id="one-time-binding-example">One time binding: {{::name}}</p>
      <p id="normal-binding-example">Normal binding: {{name}}</p>
    </div>
  </file>
  <file name="script.js">
    angular.module('oneTimeBidingExampleApp', []).
      controller('EventController', ['$scope', function($scope) {
        var counter = 0;
        var names = ['Igor', 'Misko', 'Chirayu', 'Lucas'];
        /*
         * expose the event object to the scope
         */
        $scope.clickMe = function(clickEvent) {
          $scope.name = names[counter % names.length];
          counter++;
        };
      }]);
  </file>
  <file name="protractor.js" type="protractor">
    it('should freeze binding after its value has stabilized', function() {
      var oneTimeBiding = element(by.id('one-time-binding-example'));
      var normalBinding = element(by.id('normal-binding-example'));

      expect(oneTimeBiding.getText()).toEqual('One time binding:');
      expect(normalBinding.getText()).toEqual('Normal binding:');
      element(by.buttonText('Click Me')).click();

      expect(oneTimeBiding.getText()).toEqual('One time binding: Igor');
      expect(normalBinding.getText()).toEqual('Normal binding: Igor');
      element(by.buttonText('Click Me')).click();

      expect(oneTimeBiding.getText()).toEqual('One time binding: Igor');
      expect(normalBinding.getText()).toEqual('Normal binding: Misko');

      element(by.buttonText('Click Me')).click();
      element(by.buttonText('Click Me')).click();

      expect(oneTimeBiding.getText()).toEqual('One time binding: Igor');
      expect(normalBinding.getText()).toEqual('Normal binding: Lucas');
    });
  </file>
</example>


### Why this feature

The main purpose of one-time binding expression is to provide a way to create a binding
that gets deregistered and frees up resources once the binding is stabilized.
Reducing the number of expressions being watched makes the digest loop faster and allows more
information to be displayed at the same time.


### Value stabilization algorithm

One-time binding expressions will retain the value of the expression at the end of the
digest cycle as long as that value is not undefined. If the value of the expression is set
within the digest loop and later, within the same digest loop, it is set to undefined,
then the expression is not fulfilled and will remain watched.

  1. Given an expression that starts with `::`, when a digest loop is entered and expression
     is dirty-checked, store the value as V
  2. If V is not undefined, mark the result of the expression as stable and schedule a task
     to deregister the watch for this expression when we exit the digest loop
  3. Process the digest loop as normal
  4. When digest loop is done and all the values have settled process the queue of watch
     deregistration tasks. For each watch to be deregistered check if it still evaluates
     to value that is not `undefined`. If that's the case, deregister the watch. Otherwise
     keep dirty-checking the watch in the future digest loops by following the same
     algorithm starting from step 1


### How to benefit from one-time binding

If the expression will not change once set, it is a candidate for one-time binding. 
Here are three example cases.

When interpolating text or attributes:

```html
  <div name="attr: {{::color}}">text: {{::name}}</div>
```

When using a directive with bidirectional binding and the parameters will not change:

```js
someModule.directive('someDirective', function() {
  return {
    scope: {
      name: '=',
      color: '@'
    },
    template: '{{name}}: {{color}}'
  };
});
```

```html
  <div some-directive name="::myName" color="My color is {{::myColor}}"></div>
```


When using a directive that takes an expression:

```html
<ul>
  <li ng-repeat="item in ::items">{{item.name}};</li>
</ul>
```