1-repl.md

Running JavaScript

Browser Console

The fastest way to try JavaScript is with a REPL like Node.js or a browser console. Open any page in your preferred web browser, then hit F12. Click on the Console tab and click on the prompt (marked by a blue > character). You can type JavaScript code in this field, and when you hit enter, it will be evaluated. Try typing some basic expressions.

> 2 + 2 // type this in...
// 4 the console will spit this back out

true || false;
// true

2 + 2 == 4
// true

console.log('hello world');
// hello world
// undefined

The semicolon statement terminator is optional in most all places in JS.

Node.js

Alternatively, you can write a .js file and execute it if you have Node.js installed.

// example.js
console.log('hello world')

$ node example.js

Online

Alternatively, these services let you build virtual webpages to show off code or test something small.

• JSFiddle
• repl.it
• JS Bin

Silliness

Destroy All Software famously made fun of JavaScript in a talk.

[] + [] // ''
{} + [] // 0
[] + {} // '[object Object]'

The strangeness continues with examples like >=.

0 > null  // false
0 == null // false
0 >= null // true

When in doubt, you can always find official specification of abstract behaviors. For abstract behaviors, look at the ECMA spec.

• == http://www.ecma-international.org/ecma-262/5.1/#sec-11.8.4
• >= http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

Understanding how the language is interpreted is a vital part of avoiding frustrations.