Unit testing promise based code in node.js express route/controller

If your method being tested doesn't return a promise then you can't use the promise syntax in Mocha. You can test your method the same way you'd test any other asynchronous method - with done as a parameter of it. Let's say we want to test your handler function:

var handler =  function (req, res, next) {  //...  User.findAsync(query, null, options).then(function (user) {    res.json(user);  }).catch(function (e) {    next(e);  });}

We can write a test as such:

describe("The handler", function(){     it("calls res.json", function(done){ // note the done argument         handler({query: {before: 5}, // mock request                 {json: done} // res.json calls our `done` param of this function                 function(){ throw new Error("error called"); });     });});

Note that we mocked the request, response and the next handler. Our mocked response has a json method that lets the test know it is complete (this can be a function if you want to make assertions inside it) and if next is called instead we throw to signal it's not something that was supposed to happen.