Unit testing typescript classes with jest ("Cannot read property '__extends' of null")

Question

I am trying to test class "a" (which inherits from class "b") with jest. The typescript compiler prepends the file compiled javascript of "a" with

var __extends = this.__extends || function (d, b) { ... };

unfortunately this is null, when running the unit test, so I am getting

Cannot read property '__extends' of null

I am using [email protected] to run the tests and [email protected] to compile my typescript files. Normally this would point to window, when the files are run in a browser.

Does someone know how to get around that issue?

Fenton · Accepted Answer · 2015-01-27 17:49:47Z

1

This error is not that __extends is null, it describes one of two scenarios.

Either you have forgotten to include the file that contains the base class, or you have included it after the file that contains the sub class.

Practical example:

<!-- Base class first -->
<script src="b.js"></script>
<!-- Now the sub class -->
<script src="a.js"></script>
<!-- Now things that depend on the sub class -->
<script src="tests.js"></script>

If you did this in any other order, you'd get a problem.

If you are compiling to a single file, you'll need to make sure your reference comments are in order to ensure the classes appear in the correct order in the combined file.

answered Jan 27, 2015 at 17:49

Fenton

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1 Comment

Ryan Cavanaugh Over a year ago

I think the issue is actually that jest is calling the test function using apply(null, args). The error message indicates that this is null (why jest would do that, I have no idea... {} seems like a better choice)

Nathan Bellowe · Accepted Answer · 2015-01-27 21:27:38Z

0

This error is the most common error I see when require.js isn't set up properly. Was requirejs properly loaded/configured?

Nathan

answered Jan 27, 2015 at 21:27

Nathan Bellowe

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2 Comments

david Over a year ago

this is an issue with jest ... jest is setting up it's own require function

Nathan Bellowe Over a year ago

Sorry, wasn't careful enough in my reading... thought I could help.

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