sinon: how to stub an instance method

Question

Here's the code I'm working on:

Test.js

class Test {
  constructor() {
  }

  func () {
    console.log('original')
  }
}

module.exports = Test

Mock.js

Test = require('./Test')

function Mock(){
  this.test = new Test()
}

Mock.prototype.call = function() {
  this.test.func()
}

module.exports = Mock

I'm trying to stub the Test.func call that's inside Mock.call . I've tried this:

sb = sinon.createSandbox()
sb.stub(Test, 'func').callsFake(() => { console.log('stubbed') })

But I get TypeError: Cannot stub non-existent own property func . When I do this:

sb.stub(new Test(), 'func').callsFake(() => { console.log('stubbed') })
new Mock().call()

I get original printed, meaning that the stub did not work properly. How do I stub out the function call?

Answer 1

You need to stub it on instance which you create:

var mock = new Mock();

sinon.stub(mock.test, 'func').callsFake(() => { console.log('stubbed') });
mock.call(); // should output "stubbed"