Expressions are JavaScript-like code snippets that are usually placed in bindings such as {{
expression }}
. Expressions are processed by the $parse
service. Expressions are often post processed using filters to create a more user-friendly format.
For example, these are all valid expressions in angular:
1+2
user.name
It might be tempting to think of Angular view expressions as JavaScript expressions, but that is
not entirely correct, since Angular does not use a JavaScript eval()
to evaluate expressions.
You can think of Angular expressions as JavaScript expressions with following differences:
Attribute Evaluation: evaluation of all properties are against the scope doing the
evaluation, unlike in JavaScript where the expressions are evaluated against the global
window
.
Forgiving: expression evaluation is forgiving to undefined
and null
, unlike in JavaScript,
where trying to evaluate undefined properties can generate ReferenceError
or TypeError
.
No Control Flow Statements: you cannot do any of the following in angular expression: conditionals, loops, or throw.
If, on the other hand, you do want to run arbitrary JavaScript code, you should make it a
controller method and call the method. If you want to eval()
an angular expression from
JavaScript, use the
method.$eval()
You can try evaluating different expressions here:
Evaluation of all properties takes place against a scope. Unlike JavaScript, where names default
to global window properties, Angular expressions have to use
to refer to the global $window
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).
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.
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.