How to access and test an internal (non-exports) function in a node.js module?

Question

I\'m trying to figure out on how to test internal (i.e. not exported) functions in nodejs (preferably with mocha or jasmine). And i have no idea!

Let say I have a mo

Answer 1

Working with Jasmine, I tried to go deeper with the solution proposed by Anthony Mayfield, based on rewire.

I implemented the following function (Caution: not yet thoroughly tested, just shared as a possibile strategy):

function spyOnRewired() {
    const SPY_OBJECT = "rewired"; // choose preferred name for holder object
    var wiredModule = arguments[0];
    var mockField = arguments[1];

    wiredModule[SPY_OBJECT] = wiredModule[SPY_OBJECT] || {};
    if (wiredModule[SPY_OBJECT][mockField]) // if it was already spied on...
        // ...reset to the value reverted by jasmine
        wiredModule.__set__(mockField, wiredModule[SPY_OBJECT][mockField]);
    else
        wiredModule[SPY_OBJECT][mockField] = wiredModule.__get__(mockField);

    if (arguments.length == 2) { // top level function
        var returnedSpy = spyOn(wiredModule[SPY_OBJECT], mockField);
        wiredModule.__set__(mockField, wiredModule[SPY_OBJECT][mockField]);
        return returnedSpy;
    } else if (arguments.length == 3) { // method
        var wiredMethod = arguments[2];

        return spyOn(wiredModule[SPY_OBJECT][mockField], wiredMethod);
    }
}

With a function like this you could spy on both methods of non-exported objects and non-exported top level functions, as follows:

var dbLoader = require("rewire")("../lib/db-loader");
// Example: rewired module dbLoader
// It has non-exported, top level object 'fs' and function 'message'

spyOnRewired(dbLoader, "fs", "readFileSync").and.returnValue(FULL_POST_TEXT); // method
spyOnRewired(dbLoader, "message"); // top level function

Then you can set expectations like these:

expect(dbLoader.rewired.fs.readFileSync).toHaveBeenCalled();
expect(dbLoader.rewired.message).toHaveBeenCalledWith(POST_DESCRIPTION);