How can I solve "ReferenceError: expect is not defined" error message?

Try

First, in the terminal

npm install expect.js

And in your code:

var expect = require('expect');

Mocha is a test framework; you need to provide your own assertion lib as https://mochajs.org/#assertions states. Thus, expect is indeed undefined because you never defined it.

(I recommend chai)

npm install chai

then

(see Amit Choukroune's comment pointing out to actually require chai)

then

var expect = chai.expect;