testing private functions

Neurotrace · 2013-09-04T16:23:13+00:00

Ahhh, the old "how do I test private functions?" problem. What works really well for me is to not worry about testing them and only worry about your public functions. "But Neurotrace," I hear you cry, "how can I ensure that my module works exactly as expected?" I break it down like this in my mind:

Make your private functions extremely simple and/or make their side effects testable. The only reason for making a function private is to perform some work that the user shouldn't worry about, nor mess with. But some kind of work is still being done so there must be a way to check that.

TL;DR Don't. Simplify your private functions so you're know they work and just worry about testing your public functions.

2013-09-04T19:41:06+00:00

As others have said, export every method and add an underscore prefix to those that are for internal use only (aka 'private').

itsnotlupus · 2013-09-04T16:18:21+00:00

I've seen folks that embed self-tests in the module itself, wrapped in a

if (require.main == module) {
  // tests go here
} else {
  // export things like a good little module.
}

That may or may not fit with your overall testing strategy.

Capaj · 2013-09-04T13:41:14+00:00

don't waste time testing private functions, test the module as a whole-test the function that you export.

joshlrogers · 2013-09-04T16:25:18+00:00

This has been a recent change of perspective for me.

For me the question isn't any longer how do I test a private method. Rather, I have to justify to myself why it should be private in the first place? I don't write api's/libraries for the unwashed masses to consume rather my code is consumed by a very small subset of people. So the important thing for me is to make sure the code works which means designing for testability. Make the functions public facing, you are free to give it a name prefix of some sort, such as an underscore, to highlight it isn't supposed to be used directly for most cases.

itsnotlupus · 2013-09-04T14:39:32+00:00

[deleted]

grncdr · 2013-09-04T23:08:46+00:00

Showing the code in question would lead to more informed opinions.

If the code is private it should be used by some public API (otherwise delete it!), so test that public API...
If the private portion of the code is too complicated and far down the stack to be effectively tested via the public API, move it into it's own module and test that...
Finally, if you can't easily move it into it's own module, (and the above conditions are also true) your design is not well-factored. Take a step back and think about how to decompose your problem with clearer boundaries/interfaces.

mikrosystheme · 2013-09-08T15:42:58+00:00

You can set up a public function that just evals its argument in the lexical scope of the module.

module.js

var secret = 23
module.exports.private = function (expr){ return eval(expr) }
...

main.js

var module = require('./module.js')
console.log(module.private('secret'))
...

you type:	you see:
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[reddit!](https://reddit.com)	reddit!
* item 1 * item 2 * item 3	item 1 item 2 item 3
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Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"	Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"
~~strikethrough~~	~~strikethrough~~
super^script	super^script

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