How to mock dependency classes for unit testing with mocha.js?

Question

Given that I have two ES6 classes.

This is class A:

import B from 'B';

class A {
    someFunction(){
        var dependency = new B();
        dependency.doSomething();
    }
}

And class B:

class B{
    doSomething(){
        // does something
    }
}

I am unit testing using mocha (with babel for ES6), chai and sinon, which works really great. But how can I provide a mock class for class B when testing class A?

I want to mock the entire class B (or the needed function, doesn't actually matter) so that class A doesn't execute real code but I can provide testing functionality.

This is, what the mocha test looks like for now:

var A = require('path/to/A.js');

describe("Class A", () => {

    var InstanceOfA;

    beforeEach(() => {
        InstanceOfA = new A();
    });

    it('should call B', () => {
        InstanceOfA.someFunction();
        // How to test A.someFunction() without relying on B???
    });
});

Answer 1

You can use SinonJS to create a stub to prevent the real function to be executed.

For example, given class A:

import B from './b';

class A {
    someFunction(){
        var dependency = new B();
        return dependency.doSomething();
    }
}

export default A;

And class B:

class B {
    doSomething(){
        return 'real';
    }
}

export default B;

The test could look like: ( sinon < v3.0.0 )

describe("Class A", () => {

    var InstanceOfA;

    beforeEach(() => {
        InstanceOfA = new A();
    });

    it('should call B', () => {
        sinon.stub(B.prototype, 'doSomething', () => 'mock');
        let res = InstanceOfA.someFunction();

        sinon.assert.calledOnce(B.prototype.doSomething);
        res.should.equal('mock');
    });
});

EDIT: for sinon versions >= v3.0.0, use this:

describe("Class A", () => {

    var InstanceOfA;

    beforeEach(() => {
        InstanceOfA = new A();
    });

    it('should call B', () => {
        sinon.stub(B.prototype, 'doSomething').callsFake(() => 'mock');
        let res = InstanceOfA.someFunction();

        sinon.assert.calledOnce(B.prototype.doSomething);
        res.should.equal('mock');
    });
});

You can then restore the function if necessary using object.method.restore(); :

var stub = sinon.stub(object, "method");
Replaces object.method with a stub function. The original function can be restored by calling object.method.restore(); (or stub.restore(); ). An exception is thrown if the property is not already a function, to help avoid typos when stubbing methods.