AngularJS remembers the value and compares it to a previous value. This is basic dirty-checking. If there is a change in value, then it fires the change event.

The $apply() method, which is what you call when you are transitioning from a non-AngularJS world into an AngularJS world, calls $digest(). A digest is just plain old dirty-checking. It works on all browsers and is totally predictable.

To contrast dirty-checking (AngularJS) vs change listeners (KnockoutJS and Backbone.js): While dirty-checking may seem simple, and even inefficient (I will address that later), it turns out that it is semantically correct all the time, while change listeners have lots of weird corner cases and need things like dependency tracking to make it more semantically correct. KnockoutJS dependency tracking is a clever feature for a problem which AngularJS does not have.

Issues with change listeners:

• The syntax is atrocious, since browsers do not support it natively. Yes, there are proxies, but they are not semantically correct in all cases, and of course there are no proxies on old browsers. The bottom line is that dirty-checking allows you to do POJO, whereas KnockoutJS and Backbone.js force you to inherit from their classes, and access your data through accessors.
• Change coalescence. Suppose you have an array of items. Say you want to add items into an array, as you are looping to add, each time you add you are firing events on change, which is rendering the UI. This is very bad for performance. What you want is to update the UI only once, at the end. The change events are too fine-grained.
• Change listeners fire immediately on a setter, which is a problem, since the change listener can further change data, which fires more change events. This is bad since on your stack you may have several change events happening at once. Suppose you have two arrays which need to be kept in sync for whatever reason. You can only add to one or the other, but each time you add you fire a change event, which now has an inconsistent view of the world. This is a very similar problem to thread locking, which JavaScript avoids since each callback executes exclusively and to completion. Change events break this since setters can have far-reaching consequences which are not intended and non obvious, which creates the thread problem all over again. It turns out that what you want to do is to delay the listener execution, and guarantee, that only one listener runs at a time, hence any code is free to change data, and it knows that no other code runs while it is doing so.

What about performance?

So it may seem that we are slow, since dirty-checking is inefficient. This is where we need to look at real numbers rather than just have theoretical arguments, but first let's define some constraints.

Humans are:

• Slow — Anything faster than 50 ms is imperceptible to humans and thus can be considered as "instant".

• Limited — You can't really show more than about 2000 pieces of information to a human on a single page. Anything more than that is really bad UI, and humans can't process this anyway.

So the real question is this: How many comparisons can you do on a browser in 50 ms? This is a hard question to answer as many factors come into play, but here is a test case: http://jsperf.com/angularjs-digest/6 which creates 10,000 watchers. On a modern browser this takes just under 6 ms. On Internet Explorer 8 it takes about 40 ms. As you can see, this is not an issue even on slow browsers these days. There is a caveat: The comparisons need to be simple to fit into the time limit... Unfortunately it is way too easy to add a slow comparison into AngularJS, so it is easy to build slow applications when you don't know what you are doing. But we hope to have an answer by providing an instrumentation module, which would show you which are the slow comparisons.

It turns out that video games and GPUs use the dirty-checking approach, specifically because it is consistent. As long as they get over the monitor refresh rate (typically 50-60 Hz, or every 16.6-20 ms), any performance over that is a waste, so you're better off drawing more stuff, than getting FPS higher.

Why do I have to use "{{title}}" with '@' and "title" with '='?

@ binds a local/directive scope property to the evaluated value of the DOM attribute. If you use title=title1 or title="title1", the value of DOM attribute "title" is simply the string title1. If you use title="{{title}}", the value of the DOM attribute "title" is the interpolated value of {{title}}, hence the string will be whatever parent scope property "title" is currently set to. Since attribute values are always strings, you will always end up with a string value for this property in the directive's scope when using @.

= binds a local/directive scope property to a parent scope property. So with =, you use the parent model/scope property name as the value of the DOM attribute. You can't use {{}}s with =.

With @, you can do things like title="{{title}} and then some" -- {{title}} is interpolated, then the string "and them some" is concatenated with it. The final concatenated string is what the local/directive scope property gets. (You can't do this with =, only @.)

With @, you will need to use attr.$observe('title', function(value) { ... }) if you need to use the value in your link(ing) function. E.g., if(scope.title == "...") won't work like you expect. Note that this means you can only access this attribute asynchronously. You don't need to use $observe() if you are only using the value in a template. E.g., template: '<div>{{title}}</div>'.

With =, you don't need to use $observe.

Can I also access the parent scope directly, without decorating my element with an attribute?

Yes, but only if you don't use an isolate scope. Remove this line from your directive

scope: { ... }

and then your directive will not create a new scope. It will use the parent scope. You can then access all of the parent scope properties directly.

The documentation says "Often it's desirable to pass data from the isolated scope via an expression and to the parent scope", but that seems to work fine with bidirectional binding too. Why would the expression route be better?

Yes, bidirectional binding allows the local/directive scope and the parent scope to share data. "Expression binding" allows the directive to call an expression (or function) defined by a DOM attribute -- and you can also pass data as arguments to the expression or function. So, if you don't need to share data with the parent -- you just want to call a function defined in the parent scope -- you can use the & syntax.

See also

• Lukas's isolated scope blog post (covers @, =, &)
• dnc253's explanation of @ and =
• my blog-like answer about scopes -- the directives section (way at the bottom, just before the Summary section) has a picture of an isolate scope and its parent scope -- the directive scope uses @ for one property and = for another
• What is the difference between & vs @ and = in angularJS