A javascript utility tools.

Usage no npm install needed!

<script type="module">
  import aaronflowerJtils from 'https://cdn.skypack.dev/@aaronflower/jtils';



  1. Init the project, be careful of your version.
npm init
  1. Hierarchy the project, ignore node_modules
$ mkdir src test
$ touch .gitignore
  1. Add EditorConfig Support
$ yarn add --dev editorconfig-cli
$ npx ec init
  1. Add ESLint, Prettier support
$ yarn add --dev eslint
$ npx eslint --init
$ yarn add --dev eslint-plugin-prettier eslint-config-prettier
# add .eslintrc.js .prettierrc.js config file to support.
  1. Add a utility function sum.js in src
export default function sum (a, b) {
  return number(a) + (b)

use ESLint to check out code.

npx lint src/sum.js

  1:10  error  'foo' is defined but never used  no-unused-vars
  2:10  error  Strings must use singlequote     quotes

✖ 2 problems (2 errors, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

If, you use VSCode or other editor you can install plugins for these editor to lint your code dynamically.

I use vim, so I use a vim lint plugin ALE.

  1. Add Mocha to test your code.
yarn add --dev mocha

test the code.

$  npx mocha
import sum from "../src/sum"

SyntaxError: Unexpected identifier
    at new Script (vm.js:79:7)
    at createScript (vm.js:251:10)
    at Object.runInThisContext (vm.js:303:10)

This error tells us we don't support ES6 Modules. So we need to add Babel to support.

  1. Add babel 7 support
yarn add --dev @babel/core @babel/cli @babel/preset-env
yarn add --dev @babel/register

And for Babel 7, you should add a babel.config.js

module.exports = {
  presets: ["@babel/env"]

And then execute the Mocha.

npx mocha --require @babel/register

    Two Positives
      ✓ should returns 4

  1 passing (9ms)
  1. About the test/sum.js code
const assert = require("assert")
import sum from "../src/sum"

describe("Sum", function() {
  describe("Two Positives", function() {
    it("should returns 4", function() {
      assert.equal(sum(2, 2), 4)


In the test/sum.js file, we use the Node.js built-in assert module to assert our code. Mocha allows us to use any assertion libraries we wish. This means we can use libraries such as:

  • chai Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework.
  • should.js BDD style shown throughout these docs.
  • jest/expect
  • expect.js
  • better-assert
  • unexpected

We should use Chai, Because it has several interfaces that allow the developer to choose the most comfortable.


The functions describe(), it() are Mocha's BDD "interface" system.

Mocha's "interface" system allows developers to choose thier style of DSL. Mocha has BDD, TDD, Exports, QUnit and Require-style interfaces.


The BDD is shorted for Behavior Driven Development(BDD), So we can call Mocha is a BDD supprotable Testing Frameworks.

The BDD interface provides describe(), context(), it(), specify(), before(), after(), beforeEach(), and afterEach().

context() is just an alias for describe(), and behaves the same way; it just provides a way to keep tests easier to read and organized. Similarly, sepcify() is an alias for it().

describe('Array', function() {
    before(function() {
      // ...

    describe('#indexOf()', function() {
      context('when not present', function() {
        it('should not throw an error', function() {
          (function() {
        it('should return -1', function() {
      context('when present', function() {
        it('should return the index where the element first appears in the array', function() {