In this first part of the series we will learn about node.js, npm, require, jest and pure functions.

This post is better used as reference. To understand in a more complete way please refer to Dan Abramov’s “Getting Started with Redux” course at egghead until lesson “Pure and Impure Functions”.

This post is part of the JavaScript for Grownups series:

Prerequisites

  • Node.js
  • NPM

What is Node.js?

Runtime built on Chrome’s V8 JavaScript engine.

Hello World

file: index.js

console.log('Hello World!');

Command Line

node index.js
rm index.js

NPM

Command Line

mkdir javascript-adultswim
cd javascript-adultswim

file: README.md

# javascript-adultswim

Simple node.js script

Command Line

npm init --yes

file: package.json

{
  "name": "javascript-adultswim",
  "version": "1.0.0",
  "description": "Simple node.js script",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

jest

Comand Line

npm install --save-dev jest

part of file: package.json

{
  "scripts": {
    "test": "jest"
  }
}

counter.test.js

var counter = require('./counter');

test('should return initial state if state is undefined', function() {
  expect(counter(undefined, {})).toBe(0);
});

test('increment state', function() {
    expect(counter(0, { type: 'INCREMENT' })).toBe(1);
    expect(counter(1, { type: 'INCREMENT' })).toBe(2);
});

test('decrement state', function() {
  expect(counter(0, { type: 'DECREMENT' })).toBe(-1);
  expect(counter(1, { type: 'DECREMENT' })).toBe(0);
});

test('should return current state for unknown action', function() {
  expect(counter(1, { type: 'UKNOWN_TYPE' })).toBe(1);
});

Command Line

npm test -- --watch

counter.js

module.exports = function counter(state, action) {
  if (state === undefined) return 0;
  switch (action.type) {
    case 'INCREMENT': return state + 1;
    case 'DECREMENT': return state - 1;
    default: return state;
  }
};

index.js

var counter = require('./counter');

console.log(counter(0, { type: 'INCREMENT' })); // 1
console.log(counter(1, { type: 'DECREMENT' })); // 0

Pure and impure functions

Pure function

  • no side effects (database or network calls)
  • idempotent (return value always the same for the same arguments)
  • immutable (can’t change the argument received)

ps: the previous example is pure

Impure function

  • not pure :-)