diff options
Diffstat (limited to 'docs/content/guide/expression.ngdoc')
| -rw-r--r-- | docs/content/guide/expression.ngdoc | 55 |
1 files changed, 29 insertions, 26 deletions
diff --git a/docs/content/guide/expression.ngdoc b/docs/content/guide/expression.ngdoc index 58581047..6ef972b4 100644 --- a/docs/content/guide/expression.ngdoc +++ b/docs/content/guide/expression.ngdoc @@ -2,35 +2,36 @@ @name Expressions @description -Expressions are JavaScript-like code snippets that are usually placed in bindings such as `{{ -expression }}`. Expressions are processed by the {@link ng.$parse $parse} -service. Expressions are often post processed using {@link guide/filter filters} to create a more user-friendly format. +Expressions are JavaScript-like code snippets that are usually placed in bindings such as +`{{ expression }}`. -For example, these are all valid expressions in angular: +For example, these are valid expressions in Angular: * `1+2` + * `a+b` * `user.name` + * `items[index]` -## Angular Expressions vs. JS Expressions +## Angular Expressions vs. JavaScript Expressions -It might be tempting to think of Angular view expressions as JavaScript expressions, but that is -not entirely correct, since Angular does not use a JavaScript `eval()` to evaluate expressions. -You can think of Angular expressions as JavaScript expressions with following differences: +Angular expressions are like JavaScript expressions with the following differences: - * **Attribute Evaluation:** evaluation of all properties are against the scope doing the - evaluation, unlike in JavaScript where the expressions are evaluated against the global - `window`. + * **Context:** JavaScript expressions are evaluated against the global `window`. + In Angular, expressions are evaluated against a {@link ng.$rootScope.Scope `scope`} object. - * **Forgiving:** expression evaluation is forgiving to `undefined` and `null`, unlike in JavaScript, - where trying to evaluate undefined properties can generate `ReferenceError` or `TypeError`. + * **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 do any of the following in angular expression: - conditionals, loops, or throw. + * **No Control Flow Statements:** you cannot use the following in an Angular expression: + conditionals, loops, or exceptions. -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 ng.$rootScope.Scope#$eval `$eval()`} method. + * **Filters:** You can use {@link guide/filter filters} within expressions to format data before + displaying it. + +If you want to run more complex JavaScript code, you should make it a controller method and call +the method from your view. If you want to `eval()` an Angular expression yourself, use the +{@link ng.$rootScope.Scope#$eval `$eval()`} method. ## Example <example> @@ -87,13 +88,15 @@ You can try evaluating different expressions here: </example> -# Property Evaluation +# Context + +Angular does not use JavaScript's `eval()` to evaluate expressions. Instead Angular's +{@link ng.$parse $parse} service processes these expressions. -Evaluation of all properties takes place against a scope. Unlike JavaScript, where names default -to global window properties, Angular expressions have to use {@link ng.$window -`$window`} to refer to the global `window` object. For example, if you want to call `alert()`, which is -defined on `window`, in an expression you must use `$window.alert()`. This is done intentionally to -prevent accidental access to the global state (a common source of subtle bugs). +Unlike JavaScript, where names default to global `window` properties, Angular expressions must use +{@link ng.$window `$window`} explicitly to refer to the global `window` object. For example, if you +want to call `alert()` in an expression you must use `$window.alert()`. This restriction is +intentional. It prevents accidental access to the global state – a common source of subtle bugs. <example> <file name="index.html"> @@ -142,12 +145,12 @@ It makes more sense to show nothing than to throw an exception if `a` is undefin 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. +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 +Angular philosophy that application logic should be in controllers, not the views. If you need a conditional, loop, or to throw from a view expression, delegate to a JavaScript method instead. |