My previous post showed how to set up a component’s controller for unit testing using the $componentController service. We also learned that Angular doesn’t trigger the different lifecycle hooks by itself, meaning that you need to manually call $onInit , etc.

In this post I’ll show an example for testing a component that makes use of the $onChanges hook.

Take a look at this simple word-counting component’s $onChanges hook:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 app . component ( 'wordCount' , { template : ` < div > < span ng - bind = "$ctrl.words" >< /span> words < span ng - if = "$ctrl.difference" > < span ng - bind = "$ctrl.difference" >< /span> since last save < /span> < /div> ` , bindings : { text : '<' }, controller : function () { this . $onChanges = ( changes ) => { if ( changes . text ) { this . words = countWords ( this . text ); if ( changes . text . isFirstChange ()) { this . difference = 0 ; } else { this . difference = this . words - countWords ( changes . text . previousValue ); } } }; function countWords ( text ) { var trimmed = text . trim (); if ( trimmed . length === 0 ) return 0 ; return trimmed . split ( /\s+/ ). length ; } } });

As you can see, the hook uses the changes object in order to know when the text has changes, detect the initial change, and also use the previousValue to calculate the word difference (yes, I know we could use the old words value, but this is just for the example).

You can see the component live here.

Now, testing this component is relatively trivial. We simply need to pass it some different values for text , trigger $onChanges , and that’s it. But, the fact is that Angular, for some reason, doesn’t expose the changes object it uses internally. That means that you’ll have to implement your own object that conforms to the changes object protocol–each changed property needs an isFirstChange method and 2 properties, currentValue and previousValue .

Since that’s a PITA, I’ve open sourced a tiny helper just for that, angular-stub-changes.

Using it, our tests now look like this:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 describe ( 'wordCount component' , function () { var ctrl ; beforeEach ( function () { angular . mock . module ( app ); angular . mock . inject ( function ( $componentController ) { ctrl = $componentController ( 'wordCount' , null , { text : '1 2 3' }); }); }); it ( 'counts words' , function () { var changes = new StubChanges (). addInitialChange ( 'text' , '1 2 3' ). build (); ctrl . $onChanges ( changes ); expect ( ctrl . words ). toBe ( 3 ); }); it ( 'does not show difference when initializing' , function () { var changes = new StubChanges (). addInitialChange ( 'text' , '1 2 3' ). build (); ctrl . $onChanges ( changes ); expect ( ctrl . difference ). toBe ( 0 ); }); it ( 'calculates difference when changing text' , function () { var changes = new StubChanges (). addInitialChange ( 'text' , '1 2 3' ). build (); ctrl . $onChanges ( changes ); ctrl . text = '1 2 3 4 5' ; changes = new StubChanges (). addChange ( 'text' , '1 2 3 4 5' , '1 2 3' ). build (); ctrl . $onChanges ( changes ); expect ( ctrl . difference ). toBe ( 2 ); }); });

Basically, note that we make sure to call $onChanges with a properly configured changes object. Also, you still have to set the updated properties on your controller instance yourself, e.g. ctrl.text = '...' .

The creation of the stub changes object is, I hope, straightforward. It’s a builder pattern, so you can keep on adding how many changes ( .addChange(..).addChange(..) ) as needed for your component

That’s about it, happy testing!