Stubbing and/or mocking a class in sinon.js?

Question

I've created a database wrapper for my application, shown below. To test it, I obviously would like to replace the actual database library. I could create a new class that mocks the query method and catch all input there, but using sinon.js seems more appropriate, but how would I use it?
Is the mock or stub features of sinon.js what I should be using?

wrapper = (function() {

  function wrapper() {}

  wrapper.db = require("database");

  wrapper.prototype.insertUser = function(doc) {
    return this.db.query("INSERT INTO USERS...");
  };

  return wrapper;

})();

Answer 1

First, I'd modify your class definition a bit (uppercase class name and fix db assignment):

var Wrapper = (function() {

  function Wrapper() {
    this.db = require("database");
  }

  Wrapper.prototype.insertUser = function(doc) {
    return this.db.query("INSERT INTO USERS...");
  };

  return Wrapper;

})();

To stub the whole class:

var WrapperStub = sinon.spy(function() {
  return sinon.createStubInstance(Wrapper);
});

sinon.createStubInstance will create an instance of Wrapper where every method is a stub. sinon.spy will allow us to spy the class instantiation.

So you could exercise it like this:

// verify instantiation
var wrapper = new WrapperStub();
expect(WrapperStub).to.have.been.calledWithNew;

// verify method stub
wrapper.insertUser.returns('data');
expect(wrapper.insertUser()).to.equal('data');
expect(wrapper.insertUser).to.have.been.calledOnce;

(assertions use chai and sinon-chai)

I said just "exercise it" because this code snippet is not an actual unit test. Instantiation and method calls will be made by your subject under test.

Now, if you want to mock a dependency injected by require() –such as db = require('database') in your example–, you could try a testing tool like either Jest (but not using sinon) or sinonquire which I created inspired by Jest but to use it with sinon plus your favorite testing tool (mine is mocha). Internally, sinonquire uses the same technique shown above of combining sinon.spyand sinon.createStubInstance to stub a class.