Check if the string is a valid number

function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}

Round float number

// i is 1
var i = Math.round(0.5);

// i is 1
var i = Math.ceil(0.3);

// i is 2
var i = Math.floor(2.7);

Generate random number in range

var min = 5;
var max = 10;

var randomFloat = Math.random() * (max - min) + min;

var randomInt = Math.floor(Math.random() * (max - min + 1)) + min;

Convert decimal number to binary

var dec = 255;
// bin is '11111111'
var bin = dec.toString(2);

Convert decimal number to hex

var dec = 255;
// hex is 'ff'
var hex = dec.toString(16);

Convert decimal number to oct

var dec = 255;
// oct is '377'
var oct = dec.toString(8);

Convert binary number to decimal

// h is 10
var h = parseInt("1010", 2);

Convert hex number to decimal

// h is 255
var h = parseInt("ff", 16);

Convert octal number to decimal

// h is 17
var h = parseInt("21", 8);

Euler's constant and the base of natural logarithms

Math.E

Pi constant

Math.PI

Trigonometric functions

var x = Math.PI;
Math.sin(x);
Math.cos(x);
Math.tan(x);

Math power function

// x is 8 (2**3)
var x = Math.pow(2, 3);

Calculate natural logarithm

// -1
Math.log(1/Math.E);

Find maximum number

var a = 0;
var b = -1;
var c = 10;
// m is 10
var m = Math.max(a, b, c);

Find minimum number

var a = 0;
var b = -1;
var c = 10;
// m is -1
var m = Math.max(a, b, c);

Calculate absolute value of the number

// 1.7
var a = Math.abs(-1.7);

Calculate exponent

// e is 2.718281828459045 (Math.E)
var e = Math.exp(1);