Stubbing a class method with Sinon.js

I am trying to stub a method using sinon.js but I get the following error:

Uncaught TypeError: Attempted to wrap undefined property sample_pressure as function

I also went to this question (https://stackoverflow.com/questions/12819242/stubbing-and-or-mocking-a-class-in-sinon-js) and copied and pasted the code but I get the same error.

Here is my code:

Sensor = (function() {
  // A simple Sensor class

  // Constructor
  function Sensor(pressure) {
    this.pressure = pressure;
  }

  Sensor.prototype.sample_pressure = function() {
    return this.pressure;
  };

  return Sensor;

})();

// Doesn't work
var stub_sens = sinon.stub(Sensor, "sample_pressure").returns(0);

// Doesn't work
var stub_sens = sinon.stub(Sensor, "sample_pressure", function() {return 0});

// Never gets this far
console.log(stub_sens.sample_pressure());

Here is the jsFiddle (http://jsfiddle.net/pebreo/wyg5f/5/) for the above code, and the jsFiddle for the SO question that I mentioned (http://jsfiddle.net/pebreo/9mK5d/1/).

I made sure to include sinon in the External Resources in jsFiddle and even jQuery 1.9. What am I doing wrong?

Your code is attempting to stub a function on Sensor, but you have defined the function on Sensor.prototype.

sinon.stub(Sensor, "sample_pressure", function() {return 0})

is essentially the same as this:

Sensor["sample_pressure"] = function() {return 0};

but it is smart enough to see that Sensor["sample_pressure"] doesn't exist.

So what you would want to do is something like these:

// Stub the prototype's function so that there is a spy on any new instance
// of Sensor that is created. Kind of overkill.
sinon.stub(Sensor.prototype, "sample_pressure").returns(0);

var sensor = new Sensor();
console.log(sensor.sample_pressure());

or

// Stub the function on a single instance of 'Sensor'.
var sensor = new Sensor();
sinon.stub(sensor, "sample_pressure").returns(0);

console.log(sensor.sample_pressure());

or

// Create a whole fake instance of 'Sensor' with none of the class's logic.
var sensor = sinon.createStubInstance(Sensor);
console.log(sensor.sample_pressure());

The top answer is deprecated. You should now use:

sinon.stub(YourClass.prototype, 'myMethod').callsFake(() => {
    return {}
})

Or for static methods:

sinon.stub(YourClass, 'myStaticMethod').callsFake(() => {
    return {}
})

Or for simple cases just use returns:

sinon.stub(YourClass.prototype, 'myMethod').returns({})

sinon.stub(YourClass, 'myStaticMethod').returns({})

Or if you want to stub a method for an instance:

const yourClassInstance = new YourClass();
sinon.stub(yourClassInstance, 'myMethod').returns({})

I ran into the same error trying to mock a method of a CoffeeScript class using Sinon.

Given a class like this:

class MyClass
  myMethod: ->
    # do stuff ...

You can replace its method with a spy this way:

mySpy = sinon.spy(MyClass.prototype, "myMethod")

# ...

assert.ok(mySpy.called)

Just replace spy with stub or mock as needed.

Note that you'll need to replace assert.ok with whatever assertion your testing framework has.

Thanks to @loganfsmyth for the tip. I was able to get the stub to work on an Ember class method like this:

sinon.stub(Foo.prototype.constructor, 'find').returns([foo, foo]);
expect(Foo.find()).to.have.length(2)