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

I am trying to test Javascript with mocha. I've this snippet of code:

describe('Array', function() {
    describe('indexOf()', function() {
        it("dovrebbe tornare -1 quando l'elemento non è presente", function() {

and a test/array.js file. Mocha was installed with

$ npm install -g mocha

When I run

$ mocha

I get this error:

$ mocha

0 passing (5ms)
1 failing

1) Array indexOf() dovrebbe tornare -1 quando l'elemento non è presente:
 ReferenceError: expect is not defined
  at Context.<anonymous> (/Users/simonegentili/Desktop/Javascipt Best Practice/test/array.js:4:4)
  at (/usr/local/lib/node_modules/mocha/lib/runnable.js:211:32)
  at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:358:10)
  at /usr/local/lib/node_modules/mocha/lib/runner.js:404:12
  at next (/usr/local/lib/node_modules/mocha/lib/runner.js:284:14)
  at /usr/local/lib/node_modules/mocha/lib/runner.js:293:7
  at next (/usr/local/lib/node_modules/mocha/lib/runner.js:237:23)
  at Object._onImmediate (/usr/local/lib/node_modules/mocha/lib/runner.js:261:5)
  at processImmediate [as _immediateCallback] (timers.js:317:15)



Mocha is a test framework; you need to provide your own assertion lib as states. Thus, expect is indeed undefined because you never defined it.

(I recommend chai)

npm install chai


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


var expect = chai.expect;


First, in the terminal

npm install expect.js

And in your code:

var expect = require('expect');
let chai = require('chai');

var assert = chai.assert;

describe('Array', function() {
  describe('#indexOf()', function() {
    it('should return -1 when the value is not present', function() {
      assert.equal(-1, [1, 2, 3].indexOf(4));

After you install Chai as other posts suggest, with the es6 syntax you should put the import at the top

import {expect} from 'chai';

In order to expose expect globally, while using chai, following should be loaded by means of configuration for your prefered testing library:

require('chai/register-assert');  // Using Assert style
require('chai/register-expect');  // Using Expect style
require('chai/register-should');  // Using Should style

For example:

npx mocha --config .mocharc.js *.spec.js


Install Expect.js or Chai.js if you are using Mocha for TDD

So, do npm install expect or npm install chai


In my use case, I was running a mocha spec through karma. The solution was to install the karma integrations for my test framework libs:

npm install karma-mocha --save-dev
npm install karma-sinon-chai --save-dev

...and also to add the frameworks to my karma.conf.js:

module.exports = function(config) {
        browsers: ['Chrome'],
        frameworks: ['mocha', 'sinon-chai'],
        files: [
        client: {
            chai: {
                includeStack: true
            mocha: {
                reporter: 'html',
                ui: 'bdd'

Hope this helps someone else.


Either add this script tag in html file

<script src="[email protected]%3C21/umd/expect.min.js"></script>

or install package

npm install chai or expect

Create a file chaiexpections.js and declare expect as global

'use strict';

// Configure chai
global.expect = require('chai').expect;

Now pass it in config file under cucumberopts, require

cucumberOpts: {
    require: ['./testsuites/*.js','./chaiexpections.js','./commons/hooks.js']
    //tags: [],
    strict: true,    
    format: [],  
    'dry-run': false,           
    compiler: [],
    format: 'json:results.json',   

This prevents the overhead of having to declare chai exception in each stepdefinition


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