function randomIntFromInterval(min, max) { // min and max included 
  return Math.floor(Math.random() * (max - min + 1) + min)
}

const rndInt = randomIntFromInterval(1, 6)
console.log(rndInt)

What it does "extra" is it allows random intervals that do not start with 1. So you can get a random number from 10 to 15 for example. Flexibility.

Math.random()

Returns an integer random number between min (included) and max (included):

function randomInteger(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

Or any random number between min (included) and max (not included):

function randomNumber(min, max) {
  return Math.random() * (max - min) + min;
}

Useful examples (integers):

// 0 -> 10
Math.floor(Math.random() * 11);

// 1 -> 10
Math.floor(Math.random() * 10) + 1;

// 5 -> 20
Math.floor(Math.random() * 16) + 5;

// -10 -> (-2)
Math.floor(Math.random() * 9) - 10;

** And always nice to be reminded (Mozilla):

Math.random() does not provide cryptographically secure random numbers. Do not use them for anything related to security. Use the Web Crypto API instead, and more precisely the window.crypto.getRandomValues() method.

Other solutions:

• (Math.random() * 6 | 0) + 1
• ~~(Math.random() * 6) + 1

Try online

TL;DR

function generateRandomInteger(min, max) {
  return Math.floor(min + Math.random()*(max - min + 1))
}

To get the random number generateRandomInteger(-20, 20);

EXPLANATION BELOW

integer - A number which is not a fraction; a whole number

We need to get a random number , say X between min and max. X, min and max are all integers

i.e min <= X <= max

If we subtract min from the equation, this is equivalent to

0 <= (X - min) <= (max - min)

Now, lets multiply this with a random number r which is

0 <= (X - min) * r <= (max - min) * r

Now, lets add back min to the equation

min <= min + (X - min) * r <= min + (max - min) * r

For, any given X, the above equation satisfies only when r has range of [0,1] For any other values of r the above equation is unsatisfied.

Learn more about ranges [x,y] or (x,y) here

Our next step is to find a function which always results in a value which has a range of [0,1]

Now, the range of r i.e [0,1] is very similar to Math.random() function in Javascript. Isn't it?

The Math.random() function returns a floating-point, pseudo-random number in the range [0, 1); that is, from 0 (inclusive) up to but not including 1 (exclusive)

Random Function using Math.random() 0 <= r < 1

Notice that in Math.random() left bound is inclusive and the right bound is exclusive. This means min + (max - min) * r will evaluate to having a range from [min, max)

To include our right bound i.e [min,max] we increase the right bound by 1 and floor the result.

function generateRandomInteger(min, max) {
  return Math.floor(min + Math.random()*(max - min + 1))
}

To get the random number

generateRandomInteger(-20, 20);

Or, in Underscore

_.random(min, max)

var x = 6; // can be any number
var rand = Math.floor(Math.random()*x) + 1;

jsfiddle:

Random Integer: to get a random integer between min and max, use the following code

function getRandomInteger(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min;
}

Random Floating Point Number: to get a random floating point number between min and max, use the following code

function getRandomFloat(min, max) {
  return Math.random() * (max - min) + min;
}

Reference:

Math is not my strong point, but I've been working on a project where I needed to generate a lot of random numbers between both positive and negative.

function randomBetween(min, max) {
    if (min < 0) {
        return min + Math.random() * (Math.abs(min)+max);
    }else {
        return min + Math.random() * max;
    }
}

E.g

randomBetween(-10,15)//or..
randomBetween(10,20)//or...
randomBetween(-200,-100)

Of course, you can also add some validation to make sure you don't do this with anything other than numbers. Also make sure that min is always less than or equal to max.

Get a random integer between 0 and 400

let rand = Math.round(Math.random() * 400)

document.write(rand)

Get a random integer between 200 and 1500

let range = {min: 200, max: 1500}
let delta = range.max - range.min

const rand = Math.round(range.min + Math.random() * delta)

document.write(rand)

Using functions

function randBetween(min, max){
  let delta = max - min
  return Math.round(min + Math.random() * delta)
}

document.write(randBetween(10, 15));

// JavaScript ES6 arrow function

const randBetween = (min, max) => {
  let delta = max - min
  return Math.round(min + Math.random() * delta)
}

document.write(randBetween(10, 20))

ES6 / Arrow functions version based on Francis' code (i.e. the top answer):

const randomIntFromInterval = (min, max) => Math.floor(Math.random() * (max - min + 1) + min);

I wrote more flexible function which can give you random number but not only integer.

function rand(min,max,interval)
{
    if (typeof(interval)==='undefined') interval = 1;
    var r = Math.floor(Math.random()*(max-min+interval)/interval);
    return r*interval+min;
}

var a = rand(0,10); //can be 0, 1, 2 (...) 9, 10
var b = rand(4,6,0.1); //can be 4.0, 4.1, 4.2 (...) 5.9, 6.0

Fixed version.

Example

Return a random number between 1 and 10:

Math.floor((Math.random() * 10) + 1);

The result could be: 3

Try yourself: here

--

or using lodash / undescore:

_.random(min, max)

Docs: - lodash - undescore

I was searching random number generator written in TypeScript and I have written this after reading all of the answers, hope It would work for TypeScript coders.

    Rand(min: number, max: number): number {
        return (Math.random() * (max - min + 1) | 0) + min;
    }