CODEWITHSUNDEEP
javascriptcoffeescriptmocha.js
Michael
Michael

Reputation: 3366

Mocha JS - mock parent class module

Background

I have class A that extends from class B:

A = class A
    ...
    my_inner_func: (param1) =>
        return new MyHelper()

B = class B extends A
    ...

What I want to do?

In the unittest, my_inner_func() should return MyMockHelper().

What I've tried

a = rewire './a.coffee'
b = rewire './b.coffee'

a.__set__
    MyHelper: MyMockHelper

b.__set__
    A: a.A

But B().my_inner_func() returns MyHelper instead of MyMockHelper.

My question

How does one mock a module used by an extended class in CoffeeScript (or JavaScript)?

Upvotes: 1

Views: 1210

Answers (1)

4m1r
4m1r

Reputation: 12552

The documentation for rewire doesn't say anything about class instance variables. I think its really just for "rewiring" the globals or top-level scope of the module under test.

Also, rewire has a few limitations. Among them, support for const or babel seems to be ify, so perhaps try a native solution?

At a basic level, a mock is simply overriding or replacing a method on an object's prototype, so something like this should work in your test case

// super class
class Polygon {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }

  get area() {
    return this.calcArea();
  }

  calcArea() {
    return this.height * this.width;
  }
}
// superclass instance
const rect = new Polygon(10, 2);
console.log('rect', rect.area); // 20

// subclass
class Square extends Polygon {
  constructor(side) {
    super(side, side)
  }
}

// subclass instance
const square = new Square(2);
console.log('square', square.area); // 4

// mocked calcArea of subclass
square.calcArea = function() {
    return 0;
};

// result from new mocked calcArea
console.log('mocked', square.area); // 0

using mocha might look like this...

import Square from './Square';

describe('mocks', () => {

    it('should mock calcArea', () => {

        const square = new Square(2);

        square.calcArea = function() {
            return 0;
        };

        expect(square.area).to.equal(0);
    });

});

Here's a code pen to play around with

Upvotes: 2

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