@ngdoc overview
@name Developer Guide: Expressions
@description

Expressions are JavaScript-like code snippets that are usually placed in bindings such as `{{
expression }}`. Expressions are process by the {@link api/angular.module.ng.$parse $parse}
service.

For example, these are all valid expressions in angular:

  * `1+2`
  * `3*10 | currency`
  * `user.name`


## Angular Expressions vs. JS Expressions

It might be tempting to think of angular view expressions as JavaScript expressions, but that is
not entirely correct, since angular does not use a JavaScript `eval()` to evaluate expressions.
You can think of angular expressions as JavaScript expressions with following differences
differences:

  * **Attribute Evaluation:** evaluation of all properties are against the scope, doing the
    evaluation, unlike in JavaScript where the expressions are evaluated against the global
    `window`.

  * **Forgiving:** expression evaluation is forgiving to undefined and null, unlike in JavaScript,
    where such evaluations generate `NullPointerExceptions`.

  * **No Control Flow Statements:** you cannot do any of the following in angular expression:
    conditionals, loops, or throw.

  * **Filters:** you can pass result of expression evaluations through filter chains. For example
    to convert date object into a local specific human-readable format.

If, on the other hand, you do want to run arbitrary JavaScript code, you should make it a
controller method and call the method. If you want to `eval()` an angular expression from
JavaScript, use the {@link api/angular.module.ng.$rootScope.Scope#$eval `$eval()`} method.

## Example
<doc:example>
<doc:source>
 1+2={{1+2}}
</doc:source>
<doc:scenario>
 it('should calculate expression in binding', function() {
   expect(binding('1+2')).toEqual('3');
 });
</doc:scenario>
</doc:example>

You can try evaluating different expressions here:

<doc:example>
<doc:source>
 <script>
   function Cntl2($scope) {
     var exprs = $scope.exprs = [];
     $scope.expr = '3*10|currency';
     $scope.addExp = function(expr) {
        exprs.push(expr);
     };

     $scope.removeExp = function(index) {
       exprs.splice(index, 1);
     };
   }
 </script>
 <div ng-controller="Cntl2" class="expressions">
   Expression:
   <input type='text' ng-model="expr" size="80"/>
   <button ng-click="addExp(expr)">Evaluate</button>
   <ul>
    <li ng-repeat="expr in exprs">
      [ <a href="" ng-click="removeExp($index)">X</a> ]
      <tt>{{expr}}</tt> => <span ng-bind="$parent.$eval(expr)"></span>
     </li>
   </ul>
 </div>
</doc:source>
<doc:scenario>
 it('should allow user expression testing', function() {
    element('.expressions :button').click();
    var li = using('.expressions ul').repeater('li');
    expect(li.count()).toBe(1);
    expect(li.row(0)).toEqual(["3*10|currency", "$30.00"]);
 });
</doc:scenario>
</doc:example>


# Property Evaluation

Evaluation of all properties takes place against a scope. Unlike JavaScript, where names default
to global window properties, angular expressions have to use {@link api/angular.module.ng.$window
`$window`} to refer to the global `window` object. For example, if you want to call `alert()`, which is
defined on `window`, in an expression must use `$window.alert()`. This is done intentionally to
prevent accidental access to the global state (a common source of subtle bugs).

<doc:example>
<doc:source>
 <script>
   function Cntl1($window, $scope){
     $scope.name = 'World';

     $scope.greet = function() {
       ($window.mockWindow || $window).alert('Hello ' + $scope.name);
     }
   }
 </script>
 <div class="example2" ng-controller="Cntl1">
   Name: <input ng-model="name" type="text"/>
   <button ng-click="greet()">Greet</button>
 </div>
</doc:source>
<doc:scenario>
 it('should calculate expression in binding', function() {
   var alertText;
   this.addFutureAction('set mock', function($window, $document, done) {
     $window.mockWindow = {
       alert: function(text){ alertText = text; }
     };
     done();
   });
   element(':button:contains(Greet)').click();
   expect(this.addFuture('alert text', function(done) {
     done(null, alertText);
   })).toBe('Hello World');
 });
</doc:scenario>
</doc: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

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 in the view. If you need a
conditional, loop, or to throw from a view expression, delegate to a JavaScript method instead.


## Filters

When presenting data to the user, you might need to convert the data from its raw format to a
user-friendly format. For example, you might have a data object that needs to be formatted
according to the locale before displaying it to the user. You can pass expressions through a chain
of filters like this:

       name | uppercase

The expression evaluator simply passes the value of name to {@link
api/angular.module.ng.$filter.uppercase `uppercase`} filter.

Chain filters using this syntax:

       value | filter1 | filter2

You can also pass colon-delimited arguments to filters, for example, to display the number 123
with 2 decimal points:

       123 | number:2

# The $

You might be wondering, what is the significance of the $ prefix? It is simply a prefix that
angular uses, to differentiate its API names from others. If angular didn't use $, then evaluating
`a.length()` would return undefined because neither a nor angular define such a property.

Consider that in a future version of angular we might choose to add a length method, in which case
the behavior of the expression would change. Worse yet, you the developer could create a length
property and then we would have a collision. This problem exists because angular augments existing
objects with additional behavior. By prefixing its additions with $ we are reserving our namespace
so that angular developers and developers who use angular can develop in harmony without collisions.

