CODEWITHSUNDEEP
javascriptunit-testingjestjs
DJ Forth
DJ Forth

Reputation: 1518

Using Jest, getting internal function

I am just re-looking at Jest as it's been getting a lot of good reports. However struggling to find a good way the access internal functions to test them.

So if I have:

const Add2 = (n)=> n+2;

export default (list)=>{
  return list.map(Add2());
}

Then if I was using Jasmine or Mocha I'd use rewire or babel-plugin-rewire to get the internal Add2 function like this:

var rewire = require('rewire');
var Add2 = rewire('./Adder').__get__('Add2');

it('Should add 2 to number', ()=>{
  let val = Add2(1);
  expect(val).toEqual(3);
});

However neither of them seem to work with jest and while there looks like an excellent mocking syntax I can't see any way to get internal function.

Is there a good way to do this, something I'm missing on the jest api or set up?

Upvotes: 4

Views: 7849

Answers (3)

Edo
Edo

Reputation: 3481

I was struggling with this problem for some time, and I don't think the problem is specific to Jest.

I know it's not ideal, but in many situations, I actually just decided to export the internal function, just for testing purposes:

export const Add2 = x => x + 2;

Previously, I would hate the idea of changing my code, just to make testing possible/easier. This was until I learned that this an important practice in hardware design; they add certain connection points to their piece of hardware they're designing, just so they can test whether it works properly. They are changing their design to facilitate testing.

Yes, you could totally do this with something like rewire. In my opinion, the additional complexity (and with it, mental overhead) that you introduce with such tools is not worth the payoff of having "more correct" code.

It's a trade-off, I value testing and simplicity, so for me, exporting private functions for testing purposes is fine.

Upvotes: 1

Johan Persson
Johan Persson

Reputation: 2055

You can actually achieve this if you are willing to use babel to transform your files before each test. Here's what you need to do (I'll assume you know how to get babel itself up and running, if not there are multiple tutorials available for that):

First, we need to install the babel-jest plugin for jest, and babel-plugin-rewire for babel:

npm install --save-dev babel-jest babel-plugin-rewire

Then you need to add a .babelrc file to your root directory. It should look something like this:

{
  "plugin": ["rewire"]
}

And that should be it (assuming you have babel set up correctly). babel-jest will automatically pick up the .babelrc, so no additional config needed there unless you have other transforms in place already. Babel will transform all the files before jest runs them, and speedskater's rewire plugin will take care of exposing the internals of your modules via the rewire API.

Upvotes: 2

Andreas Köberle
Andreas Köberle

Reputation: 110892

This is not possible with jest. Also you should not test the internals of a module, but only the public API, cause this is what other module consume. They don't care how Add2 is implemented as long as yourModule([1,2,3]) returns [3,4,5].

Upvotes: -2

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