Unit testing of private functions with Mocha and Node.js

Question

I am using Mocha in order to unit test an application written for Node.js.

I wonder if it's possible to unit test functions that have not been exported in a module.

Example:

I have a lot of functions defined like this in foobar.js:

function private_foobar1(){
    ...
}

function private_foobar2(){
    ...
}

And a few functions exported as public:

exports.public_foobar3 = function(){
    ...
}

The test case is structured as follows:

describe("private_foobar1", function() {
    it("should do stuff", function(done) {
        var stuff = foobar.private_foobar1(filter);
        should(stuff).be.ok;
        should(stuff).....

Obviously this does not work, since private_foobar1 is not exported.

What is the correct way to unit-test private methods? Does Mocha have some built-in methods for doing that?

Answer 1

As is almost always true, I think the best answer to this question is architectural: write your code in a way that is NATIVELY testable, without requiring additional dependencies that won't go into production.

If you are writing ES6 code, then your default ES6 module export should be your ES6 source. You shouldn't NEED transpiling.

In this case, do the following:

• Create a "private" class whose testable methods are public (therefore accessible to your unit test test framework) but are NOT exported from your package.

• Include this class by composition (not inheritance) into a "public" wrapper class that IS exported from your package.

Now you have the best possible scenario:

• You can test any and every class method AS IS, without adding any new dependencies or modifying the tested code before publication.
• You have complete control over which methods the wrapper class exposes.

Answer 2

As an option, create a duplicate code with an injection.

Example:

./prod_code.js

export default class A{
  #privateMethod(){
     return 'hello';
  }
}

./test_code.js

    import A from './prod_code.js';

    function inject_method_into_duplicate_сlass(MClass,injectMethodStr){
        let str_Class = MClass.toString();
        let code='return '+MClass.toString().replace (/^[\s]*class[\s]+(\w+)([\s]+extends[\s]+[\w]+)?[\s]*{([\s\S]*)}[\s]*$/,function(str,class_name,extend_class,code){
            return `class ${class_name}${extend_class??''} {\n${injectMethodStr} ${code}}`;
        });
        return Function(code)();
    }

//...
    let Mod_A=inject_method_into_duplicate_сlass(A,'static runPrivateMethod(name,...args){return eval(`this.${name}`)(...args);}')

    assert.ok(Mod_A.runPrivateMethod('#privateMethod')==='hello');

The code is provided as an example. Everyone can come up with their own implementation for the test.

With the help of such injections, the product code will be as clean as possible from the test code.

Updated

But this method has a side effect - all methods, properties and class data initialized behind the class will not be present in the duplicate. therefore, you will have to initialize such properties and methods yourself.

Example

class A{
}
//initialization behind  class
A.prop='hello';
Object.defineProprty(A,'prop2',{
  value:'bay'
});

Updated

There will also be problems with declaring global variables or modules in a class.

import A from './A.js';
 class B extends A // when creating a duplicate, class A must be in scope
{
}
// or 
class B{
   #privateMethod(){
      return new A();// when creating a duplicate, class A must be in scope
   }
}

But such difficulties are solvable.

1. you create a test file like tests/dublicate_your_class.js
2. declare the dependencies of the main class in it
3. dynamically duplicate the class with the necessary injections
4. Synchronize the properties, methods, and data of the duplicate class with the main class.
5. Use a duplicate class throughout unit testing.

But the only problem left is if the main class has side effects. Those. if its static data changes during the execution of other code. But in this case, testing such code is not within the purview of unit testing.

Answer 3

I know that this is not necessarily the answer you are looking for, but I have found that most of the time if a private function is worth testing, it's worth being in its own file.

E.g., instead of having private methods in the same file as the public ones, like this...

src/thing/PublicInterface.js

function helper1 (x) {
    return 2 * x;
}

function helper2 (x) {
    return 3 * x;
}

export function publicMethod1(x) {
    return helper1(x);
}

export function publicMethod2(x) {
    return helper1(x) + helper2(x);
}

...you split it up like this:

src/thing/PublicInterface.js

import {helper1} from './internal/helper1.js';
import {helper2} from './internal/helper2.js';

export function publicMethod1(x) {
    return helper1(x);
}

export function publicMethod2(x) {
    return helper1(x) + helper2(x);
}

src/thing/internal/helper1.js

export function helper1 (x) {
    return 2 * x;
}

src/thing/internal/helper2.js

export function helper2 (x) {
    return 3 * x;
}

That way, you can easily test helper1 and helper2 as-is, without using Rewire and other "magic" (which, I have found, have their own pain points while debugging, or when you try to make your move towards TypeScript, not to mention poorer understandability for new colleagues). And them being in a sub-folder called internal, or something like that, will help avoiding accidental usage of them in unintended places.

---

P.S.: Another common issue with "private" methods is that if you want to test publicMethod1 and publicMethod2 and mock the helpers, again, you normally need something like Rewire to do that. However, if they are in separate files, you can use Proxyquire to do it, which, unlike Rewire, doesn't need any changes to your build process, is easy to read and to debug, and works well even with TypeScript.

Answer 4

I followed barwin's answer and checked how unit tests can be made with rewire module. I can confirm that this solution simply works.

The module should be required in two parts - a public one and a private one. For public functions you can do that in standard way:

const { public_foobar3 } = require('./foobar');

For private scope:

const privateFoobar = require('rewire')('./foobar');
const private_foobar1 = privateFoobar .__get__('private_foobar1');
const private_foobar2 = privateFoobar .__get__('private_foobar2');

In order to know more about the subject, I created a working example with full module testing, testing includes private and public scope.

For further information I encourage you to check the article (How to test private functions of a CommonJS module) fully describing the subject. It includes code samples.

Answer 5

Here is a really good workflow to test your private methods explained by Philip Walton, a Google engineer on his blog.

Principle

• Write your code normally
• Bind your private methods to the object in a separate code block, and mark it by an _ (for example)
• Surround that code block by start and end comments

Then use a build task or your own build system (for example grunt-strip-code) to strip this block for production builds.

Your tests builds have access to your private API, and your production builds have not.

Snippet

Write your code as this:

var myModule = (function() {

  function foo() {
    // Private function `foo` inside closure
    return "foo"
  }

  var api = {
    bar: function() {
      // Public function `bar` returned from closure
      return "bar"
    }
  }

  /* test-code */
  api._foo = foo
  /* end-test-code */

  return api
}())

And your Grunt tasks like this:

grunt.registerTask("test", [
  "concat",
  "jshint",
  "jasmine"
])
grunt.registerTask("deploy", [
  "concat",
  "strip-code",
  "jshint",
  "uglify"
])

Deeper

In a later article, it explains the "why" of "testing private methods"

Answer 6

I made an npm package for this purpose that you might find useful: require-from

Basically, you expose non-public methods by:

module.testExports = {
    private_foobar1: private_foobar1,
    private_foobar2: private_foobar2,
    ...
}

Note: testExports can be any valid name you want, except exports of course.

And from another module:

var requireFrom = require('require-from');
var private_foobar1 = requireFrom('testExports', './path-to-module').private_foobar1;

Answer 7

If the function is not exported by the module, it cannot be called by test code outside the module. That's due to how JavaScript works, and Mocha cannot by itself circumvent this.

In the few instances where I determined that testing a private function is the right thing to do, I've set some environment variable that my module checks to determine whether it is running in a test setup or not. If it runs in the test setup, then it exports additional functions that I can then call during testing.

The word "environment" is loosely used here. It might mean checking process.env or something else that can communicate to the module "you're being tested now". The instances where I've had to do this were in a RequireJS environment, and I've used module.config for this purpose.

Answer 8

To make private methods available for testing, I do this:

const _myPrivateMethod: () => {};

const methods = {
    myPublicMethod1: () => {},
    myPublicMethod2: () => {},
}

if (process.env.NODE_ENV === 'test') {
    methods._myPrivateMethod = _myPrivateMethod;
}

module.exports = methods;

Answer 9

I have added an extra function that I name Internal() and return all private functions from there. This Internal() function is then exported. Example:

function Internal () {
  return { Private_Function1, Private_Function2, Private_Function2}
}

// Exports --------------------------
module.exports = { PublicFunction1, PublicFunction2, Internal }

You can call the internal functions like this:

let test = require('.....')
test.Internal().Private_Function1()

I like this solution best because:

• only one function Internal() is always exported. This Internal() function is always used to test private functions.
• It is simple to implement
• Low impact on the production code (only one extra function)

Answer 10

If you'd prefer to keep it simple, just export the private members as well, but clearly separated from the public API with some convention, e.g. prefix them with an _ or nest them under a single private object.

var privateWorker = function() {
    return 1
}

var doSomething = function() {
    return privateWorker()
}

module.exports = {
    doSomething: doSomething,
    _privateWorker: privateWorker
}

Answer 11

Check out the rewire module. It allows you to get (and manipulate) private variables and functions within a module.

So in your case the usage would be something like:

var rewire = require('rewire'),
    foobar = rewire('./foobar'); // Bring your module in with rewire

describe("private_foobar1", function() {

    // Use the special '__get__' accessor to get your private function.
    var private_foobar1 = foobar.__get__('private_foobar1');

    it("should do stuff", function(done) {
        var stuff = private_foobar1(filter);
        should(stuff).be.ok;
        should(stuff).....