216 lines
9.7 KiB
HTML
216 lines
9.7 KiB
HTML
<h1><code ng:non-bindable=""></code>
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<span class="hint"></span>
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</h1>
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<div><a href="http://github.com/angular/angular.js/edit/master/docs/content/guide/expression.ngdoc" class="improve-docs btn btn-primary">Improve this doc</a><p>Expressions are JavaScript-like code snippets that are usually placed in bindings such as <code>{{
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expression }}</code>. Expressions are processed by <a href="api/ng.$parse"><code>$parse</code></a>
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service.</p>
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<p>For example, these are all valid expressions in angular:</p>
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<ul>
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<li><code>1+2</code></li>
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<li><code>3*10 | currency</code></li>
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<li><code>user.name</code></li>
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</ul>
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<h3>Angular Expressions vs. JS Expressions</h3>
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<p>It might be tempting to think of Angular view expressions as JavaScript expressions, but that is
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not entirely correct, since Angular does not use a JavaScript <code>eval()</code> to evaluate expressions.
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You can think of Angular expressions as JavaScript expressions with following differences:</p>
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<ul>
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<li><p><strong>Attribute Evaluation:</strong> evaluation of all properties are against the scope, doing the
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evaluation, unlike in JavaScript where the expressions are evaluated against the global
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<code>window</code>.</p></li>
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<li><p><strong>Forgiving:</strong> expression evaluation is forgiving to undefined and null, unlike in JavaScript,
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where such evaluations generate <code>NullPointerExceptions</code>.</p></li>
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<li><p><strong>No Control Flow Statements:</strong> you cannot do any of the following in angular expression:
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conditionals, loops, or throw.</p></li>
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<li><p><strong>Filters:</strong> you can pass result of expression evaluations through filter chains. For example
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to convert date object into a local specific human-readable format.</p></li>
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</ul>
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<p>If, on the other hand, you do want to run arbitrary JavaScript code, you should make it a
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controller method and call the method. If you want to <code>eval()</code> an angular expression from
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JavaScript, use the <a href="api/ng.$rootScope.Scope#$eval"><code><code>$eval()</code></code></a> method.</p>
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<h3>Example</h3>
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<h3>Source</h3>
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<div source-edit="" source-edit-deps="angular.js" source-edit-html="index.html-60" source-edit-css="" source-edit-js="" source-edit-unit="" source-edit-scenario="scenario.js-61"></div>
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<div class="tabbable"><div class="tab-pane" title="index.html">
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<pre class="prettyprint linenums" ng-set-text="index.html-60" ng-html-wrap=" angular.js"></pre>
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<script type="text/ng-template" id="index.html-60">
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1+2={{1+2}}
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</script>
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</div>
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<div class="tab-pane" title="End to end test">
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<pre class="prettyprint linenums" ng-set-text="scenario.js-61"></pre>
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<script type="text/ng-template" id="scenario.js-61">
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it('should calculate expression in binding', function() {
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expect(binding('1+2')).toEqual('3');
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});
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</script>
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</div>
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</div><h3>Demo</h3>
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<div class="well doc-example-live animator-container" ng-embed-app="" ng-set-html="index.html-60" ng-eval-javascript=""></div>
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<p>You can try evaluating different expressions here:</p>
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<h3>Source</h3>
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<div source-edit="" source-edit-deps="angular.js script.js" source-edit-html="index.html-63" source-edit-css="" source-edit-js="script.js-62" source-edit-unit="" source-edit-scenario="scenario.js-64"></div>
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<div class="tabbable"><div class="tab-pane" title="index.html">
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<pre class="prettyprint linenums" ng-set-text="index.html-63" ng-html-wrap=" angular.js script.js"></pre>
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<script type="text/ng-template" id="index.html-63">
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<div ng-controller="Cntl2" class="expressions">
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Expression:
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<input type='text' ng-model="expr" size="80"/>
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<button ng-click="addExp(expr)">Evaluate</button>
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<ul>
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<li ng-repeat="expr in exprs">
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[ <a href="" ng-click="removeExp($index)">X</a> ]
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<tt>{{expr}}</tt> => <span ng-bind="$parent.$eval(expr)"></span>
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</li>
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</ul>
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</div>
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</script>
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</div>
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<div class="tab-pane" title="script.js">
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<pre class="prettyprint linenums" ng-set-text="script.js-62"></pre>
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<script type="text/ng-template" id="script.js-62">
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function Cntl2($scope) {
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var exprs = $scope.exprs = [];
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$scope.expr = '3*10|currency';
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$scope.addExp = function(expr) {
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exprs.push(expr);
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};
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$scope.removeExp = function(index) {
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exprs.splice(index, 1);
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};
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}
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</script>
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</div>
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<div class="tab-pane" title="End to end test">
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<pre class="prettyprint linenums" ng-set-text="scenario.js-64"></pre>
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<script type="text/ng-template" id="scenario.js-64">
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it('should allow user expression testing', function() {
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element('.expressions :button').click();
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var li = using('.expressions ul').repeater('li');
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expect(li.count()).toBe(1);
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expect(li.row(0)).toEqual(["3*10|currency", "$30.00"]);
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});
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</script>
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</div>
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</div><h3>Demo</h3>
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<div class="well doc-example-live animator-container" ng-embed-app="" ng-set-html="index.html-63" ng-eval-javascript="script.js-62"></div>
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<h2>Property Evaluation</h2>
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<p>Evaluation of all properties takes place against a scope. Unlike JavaScript, where names default
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to global window properties, Angular expressions have to use <a href="api/ng.$window"><code><code>$window</code></code></a> to refer to the global <code>window</code> object. For example, if you want to call <code>alert()</code>, which is
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defined on <code>window</code>, in an expression you must use <code>$window.alert()</code>. This is done intentionally to
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prevent accidental access to the global state (a common source of subtle bugs).</p>
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<h3>Source</h3>
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<div source-edit="" source-edit-deps="angular.js script.js" source-edit-html="index.html-66" source-edit-css="" source-edit-js="script.js-65" source-edit-unit="" source-edit-scenario="scenario.js-67"></div>
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<div class="tabbable"><div class="tab-pane" title="index.html">
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<pre class="prettyprint linenums" ng-set-text="index.html-66" ng-html-wrap=" angular.js script.js"></pre>
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<script type="text/ng-template" id="index.html-66">
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<div class="example2" ng-controller="Cntl1">
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Name: <input ng-model="name" type="text"/>
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<button ng-click="greet()">Greet</button>
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</div>
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</script>
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</div>
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<div class="tab-pane" title="script.js">
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<pre class="prettyprint linenums" ng-set-text="script.js-65"></pre>
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<script type="text/ng-template" id="script.js-65">
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function Cntl1($window, $scope){
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$scope.name = 'World';
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$scope.greet = function() {
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($window.mockWindow || $window).alert('Hello ' + $scope.name);
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}
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}
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</script>
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</div>
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<div class="tab-pane" title="End to end test">
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<pre class="prettyprint linenums" ng-set-text="scenario.js-67"></pre>
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<script type="text/ng-template" id="scenario.js-67">
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it('should calculate expression in binding', function() {
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var alertText;
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this.addFutureAction('set mock', function($window, $document, done) {
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$window.mockWindow = {
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alert: function(text){ alertText = text; }
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};
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done();
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});
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element(':button:contains(Greet)').click();
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expect(this.addFuture('alert text', function(done) {
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done(null, alertText);
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})).toBe('Hello World');
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});
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</script>
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</div>
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</div><h3>Demo</h3>
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<div class="well doc-example-live animator-container" ng-embed-app="" ng-set-html="index.html-66" ng-eval-javascript="script.js-65"></div>
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<h3>Forgiving</h3>
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<p>Expression evaluation is forgiving to undefined and null. In JavaScript, evaluating <code>a.b.c</code> throws
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an exception if <code>a</code> is not an object. While this makes sense for a general purpose language, the
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expression evaluations are primarily used for data binding, which often look like this:</p>
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<pre><code> {{a.b.c}}
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</code></pre>
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<p>It makes more sense to show nothing than to throw an exception if <code>a</code> is undefined (perhaps we are
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waiting for the server response, and it will become defined soon). If expression evaluation wasn't
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forgiving we'd have to write bindings that clutter the code, for example: <code>{{((a||{}).b||{}).c}}</code></p>
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<p>Similarly, invoking a function <code>a.b.c()</code> on undefined or null simply returns undefined.</p>
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<h3>No Control Flow Statements</h3>
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<p>You cannot write a control flow statement in an expression. The reason behind this is core to the
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Angular philosophy that application logic should be in controllers, not in the view. If you need a
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conditional, loop, or to throw from a view expression, delegate to a JavaScript method instead.</p>
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<h3>Filters</h3>
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<p>When presenting data to the user, you might need to convert the data from its raw format to a
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user-friendly format. For example, you might have a data object that needs to be formatted
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according to the locale before displaying it to the user. You can pass expressions through a chain
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of filters like this:</p>
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<pre><code> name | uppercase
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</code></pre>
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<p>The expression evaluator simply passes the value of name to <a href="api/ng.filter:uppercase"><code><code>uppercase</code></code></a> filter.</p>
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<p>Chain filters using this syntax:</p>
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<pre><code> value | filter1 | filter2
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</code></pre>
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<p>You can also pass colon-delimited arguments to filters, for example, to display the number 123
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with 2 decimal points:</p>
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<pre><code> 123 | number:2
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</code></pre>
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<h2>The $</h2>
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<p>You might be wondering, what is the significance of the $ prefix? It is simply a prefix that
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angular uses, to differentiate its API names from others. If angular didn't use $, then evaluating
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<code>a.length()</code> would return undefined because neither a nor angular define such a property.</p>
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<p>Consider that in a future version of Angular we might choose to add a length method, in which case
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the behavior of the expression would change. Worse yet, you the developer could create a length
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property and then we would have a collision. This problem exists because Angular augments existing
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objects with additional behavior. By prefixing its additions with $ we are reserving our namespace
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so that angular developers and developers who use Angular can develop in harmony without collisions.</p></div>
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