@description = "This lesson is a whirlwind tour of JavaScript features for people already fluent in another programming language. For a more measured and beginner-focused tutorial, start at the top of this Learn to Code with JavaScript track instead."

JavaScript for Programmers

"In JavaScript, there is a beautiful, elegant, highly expressive language that is buried under a steaming pile of good intentions and blunders." - Douglas Crockford, JavaScript: The Good Parts

Types

JavaScript has seven data types; six are called primitives:

• Boolean
• Null
• Undefined
• Number
• String
• Symbol (new in ECMAScript 6)

Object (sometimes called "hash") comprises all other types, including

• RegExp
• Function
• Date
• Map
• etc.

MDN: JS Data Structures

Objects

• JS Object acts as a Hash (aka Map or Dictionary)
• Keys are converted into strings
• Values can be any type

Properties

• Properties are values stored on an object
• Accessed two ways:
  1. dot user.name
  2. bracket user['name']

Arrays

Looping over the elements of an array

The normal way:

for (var i = 0; i<a.length; ++i) { ... }

In old JavaScript implementations, the above suffers from a performance problem, so you may see the following recommended instead:

    for (var i = 0, n = a.length; i<n; ++i) { ... }

Equality

• == performs type coercion
• this leads to some improbable results

2 == [2] // true

The above evaluates as if it were expanded thus:

2 === Number([2].valueOf().toString())

see also Why does 2 == [2] in JavaScript?

More equality madness

""           ==   "0"           // false
0            ==   ""            // true
0            ==   "0"           // true
false        ==   "false"       // false
false        ==   "0"           // true
false        ==   undefined     // false
false        ==   null          // false
null         ==   undefined     // true
" \t\r\n"    ==   0             // true

(from http://bonsaiden.github.com/JavaScript-Garden/#types.equality)

Moral: always use === (triple equal)

Resources

General JavaScript Resources

• Mozilla JavaScript Guide
• http://jibbering.com/faq/
• http://jibbering.com/faq/notes/
• http://www.crockford.com/javascript/survey.html
• JavaScript Garden - "a growing collection of documentation about the most quirky parts of the JavaScript programming language. It gives advice to avoid common mistakes, subtle bugs, as well as performance issues and bad practices that non-expert JavaScript programmers may encounter on their endeavours into the depths of the language."