Question

I need to make sure that a certain method inside the below shown UserMock-class was called. I’ve created this mock version to inject into another module to prevent default behaviour during testing.

I am already using sinon.js, so how can I access a method such as isValid() and replace it with a spy/stub? Is it possible to do this without instantiating the class?

var UserMock = (function() {
  var User;
  User = function() {};
  User.prototype.isValid = function() {};
  return User;
})();

Thank you

Answer 1

var UserMock = (function() {
  var User;
  User = function() {};
  User.prototype.isValid = function() {};
  return User;
})();

---

Simply via prototype:

(function(_old) {
    UserMock.prototype.isValid = function() {
        // my spy stuff
        return _old.apply(this, arguments); // Make sure to call the old method without anyone noticing 
    }
})(UserMock.prototype.isValid);

---

Explanation:

(function(_old) {

and

})(UserMock.prototype.isValid);

Makes a reference to the method isValue to the variable _old. The closure is made so we don’t pulede the parent scope with the variable.

UserMock.prototype.isValid = function() {

Redeclares the prototype method

return _old.apply(this, arguments); // Make sure to call the old method without anyone noticing 

Calling the old method and returning the result from it.

Using apply lets put in the right scope (this) with all the arguments passed to the function
Eg. if we make a simple function and apply it.

function a(a, b, c) {
   console.log(this, a, b, c);
}

//a.apply(scope, args[]);
a.apply({a: 1}, [1, 2, 3]);

a(); // {a: 1}, 1, 2, 3