nodejs override a function in a module

Question

I am trying to test a function in a module. This function ( I will refer to it as function_a ) calls a different function ( function_b ) within the same file. So this module looks like this:

//the module file

module.exports.function_a = function (){ 
  //does stuff
  function_b()
};

module.exports.function_b = function_b = function () {
  //more stuff
}

I need to test function_a with a specific result from function_b.

I would like to override function_b from my test file, then call function_a from my test file, resulting in function_a calling this override function instead of function_b.

Just a note, I have tried and succeeded in overriding functions from separate modules, like this question, but that is not what I am interested in.

I have tried the code below, and as far as I know, doesn't work. It does illustrates what I am going for, though.

//test file
that_module = require("that module")
that_module.function_b = function () { ...override ... }
that_module.function_a() //now uses the override function

Is there a correct way to do this?

Answer 1

From outside a module's code, you can only modify that module's exports object. You can't "reach into" the module and change the value of function_b within the module code. However, you can (and did, in your final example) change the value of exports.function_b.

If you change function_a to call exports.function_b instead of function_b, your external change to the module will happen as expected.

Answer 2

You can actually use the package rewire. It allows you to get and set whatever was declared in the module

foo.js

const _secretPrefix = 'super secret ';

function secretMessage() {
    return _secretPrefix + _message();
}

function _message() {
    return 'hello';
}

foo.test.js

const rewire = require('rewire');

// Note that the path is relative to `foo.test.js`
const fooRewired = rewire('path_to_foo');

// Outputs 'super secret hello'
fooRewired.secretMessage();

fooRewired.__set__('_message', () => 'ciao')

// Outputs 'super secret ciao'
fooRewired.secretMessage();