JavaScript Basics

var a;                          # variable
var b = "init";                 # string
var c = "Hi" + " " + "Joe";     # = "Hi Joe"
var d = 1 + 2 + "3";            # = "33"
var e = [2,3,5,8];              # array
var f = false;                  # boolean
var g = /()/;                   # RegEx
var h = function(){};           # function object
const PI = 3.14;                # constant
var a = 1, b = 2, c = a + b;    # one line
let z = 'zzz';                  # block scope local

var age = 18;                           # number
var name = "Jane";                      # string
var obj = {first:"Jane", last:"Doe"};  # object
var truth = false;                      # boolean
var arr = ["HTML","CSS","JS"];       # array
var a; typeof a;                        # undefined
var a = null;                           # value null

a = b + c - d;      # addition, subtraction
a = b * (c / d);    # multiplication, division
x = 100 % 48;       # modulo. 100 / 48 remainder = 4
a++; b--;           # postfix increment and decrement
a == b              # equals
a === b             # strict equal
a !== b             # strict unequal
a != b              # not equal
a < b   a > b       # less and greater than
a <= b  a >= b      # less or equal, greater or equal
a += b              # a = a + b
a && b              # logical and
a || b              # logical or
typeof a            # type of variable

"use strict";   # Use strict mode to write secure code
x = 1;          # Throws an error (variable not declared)

console.log(a);             # write to the browser console
document.write(a);          # write to the HTML
alert(a);                   # output in an alert box
confirm("Really?");         # yes/no dialog, returns true/false
prompt("Your age?","0");    # input dialog. Second parameter is default

/* Multi line
   comment */

# One line comment

<script type="text/javascript">
  # Your code here
</script>

# Include external JS file
<script src="filename.js"></script>