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.