What the hell is Ember.K?

Sometimes you’ll be looking through a code base, or even the Ember source code itself, and you’ll come across a weird looking snippet like this:

onRegister: Ember.K

What is Ember.K, why is it being used and should you use it?

Spelunking the Ember.js source

Grab the source if you don’t have it.

I use The Silver Searcher for digging around code bases, and highly recommend it. Let’s run it against the Ember codebase, looking for Ember.K:

~/code/oss/ember.js master>  ag 'Ember.K'

You’ll get just shy of 100 lines of output back, showing you that Ember.K is used a fair amount throughout the codebase. For example:

196:Ember.K = K;
201:if ('undefined' === typeof Ember.assert) { Ember.assert = Ember.K; }
202:if ('undefined' === typeof Ember.warn) { Ember.warn = Ember.K; }
203:if ('undefined' === typeof Ember.debug) { Ember.debug = Ember.K; }
204:if ('undefined' === typeof Ember.runInDebug) { Ember.runInDebug = Ember.K; }
205:if ('undefined' === typeof Ember.deprecate) { Ember.deprecate = Ember.K; }

So we’ve found it in use, but what is it doing? You might be able to deduce its purpose already, but lets take a look at the definition first.

Finding the source

Let’s run The Silver Searcher again, but this time looking for the definition of Ember.K:

~/code/oss/ember.js master>  ag 'Ember.K = '
196:Ember.K = K;

It looks like the definition lives in packages/ember-metal/lib/core.js. Right enough, the definition is present:

  Empty function. Useful for some operations. Always returns `this`.

  @method K
  @return {Object}
var K = function() { return this; };
export var K = K;
Ember.K = K;

The comment is vague: ‘Empty function. Useful for some operations. Always returns this‘. It’s a function that returns this – what use is that?

Dissecting Ember.K and its usage

We know from our explorations that Ember.K returns this. Why would we want to do that?

You probably noticed in core.js definitions like:

if ('undefined' === typeof Ember.assert) { Ember.assert = Ember.K; }

From this we can see that Ember.K is being used as a safe way to neutralize the side effects of any calls to Ember.assert when it is undefined. The line of code above essentially makes calls to Ember.assert ‘safe’, because in the worst case scenario this will be returned, which is guaranteed to be the containing object’s instance.

If we take a look at packages/ember-views/lib/views/view.js and search for willInsertElement, we can see that it also has been assigned Ember.K. In this instance, Ember.K is being used to safely create the hook that views provide to subclasses, again avoiding the caller experiencing an undefined value being returned as a result.

In this situation, Ember.K is also a truthy value, so that when the function result is used in a conditional, it evaluates as truthy, allowing the default hook to be called and provide a useful value inside the conditional.

You can use this technique when writing views or components, where you have written some template code that raises events, but you aren’t quite sure what those events should do.

For example, imagine writing the template:

  <a {{action 'someEvent'}}>Raise Event</a>

But you don’t know what someEvent should do yet. Clicking on the element will result in an error in the Console:

[Error] Error: Nothing handled the action 'someEvent'. If you did handle the action, this error can be caused by returning true from an action handler in a controller, causing the action to bubble.

By simply defining the action (on the Route, for arguments sake) with Ember.K, you can satisfy Ember.js and avoid the error:

actions: {
  someEvent: Ember.K

You can continue writing your template and deal with the event logic later.


But why is the function called K? I don’t know for sure, but it might be a nod to the K Combinator which is a function in Combinatorial Logic that simply returns its first argument. Here, we are returning this which could be said the be the secret first argument of every Javascript method call.

In short, you can use Ember.K for:

I hope that’s been helpful! If you have any corrections, or questions don’t hesitate to email me.