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Understanding Math.random()

Math.random() generates a random decimal number between 0 (inclusive) and 1 (exclusive). This is the foundation for creating all kinds of random values in your programs!

Basic Usage

⚡ javascript

// Generates a random number: 0 <= number < 1
console.log(Math.random())  // 0.7234891...
console.log(Math.random())  // 0.3891234...
console.log(Math.random())  // 0.9123456...
 // Each time you call it, you get a different number!
console.log(Math.random())  // 0.1567890...

Key Points:

Returns a decimal between 0 and 0.999...
Never returns exactly 1
Different every time you call it
Perfect for games, simulations, and randomness

Visual Range:

Math.random() can be anywhere here

Includes 0, but never quite reaches 1

Generating Random Integers

Usually we want whole numbers, not decimals. Let's learn how to generate random integers!

Random Number 0-9

⚡ javascript

// Multiply by 10, then round down
const random = Math.floor(Math.random() * 10)
console.log(random)  // 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9
 // How it works:
// Math.random()    →  0.7234...
// * 10             →  7.234...
// Math.floor()     →  7

Random Number 1-10

⚡ javascript

// Add 1 to shift the range
const random = Math.floor(Math.random() * 10) + 1
console.log(random)  // 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10
 // How it works:
// Math.random()    →  0.3891...
// * 10             →  3.891...
// Math.floor()     →  3
// + 1              →  4

The Pattern

⚡ javascript

// Random number from 0 to N-1:
Math.floor(Math.random() * N)
 // Random number from 1 to N:
Math.floor(Math.random() * N) + 1
 // Random number from 0 to 99:
Math.floor(Math.random() * 100)
 // Random number from 1 to 100:
Math.floor(Math.random() * 100) + 1

Random Numbers in a Range

What if you want a random number between 50 and 100? Or 10 and 20? Here's the formula!

The Universal Formula

⚡ javascript

// Random number between min and max (inclusive)
function randomRange(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min
}
 console.log(randomRange(50, 100))  // Between 50 and 100
console.log(randomRange(10, 20))   // Between 10 and 20
console.log(randomRange(1, 6))     // Dice roll!

Breaking Down the Formula

⚡ javascript

// Example: Random number between 50 and 100
const min = 50
const max = 100
 // Step by step:
const range = max - min + 1        // 100 - 50 + 1 = 51
const random = Math.random()       // 0.7234...
const scaled = random * range      // 0.7234... * 51 = 36.8934...
const floored = Math.floor(scaled) // 36
const shifted = floored + min      // 36 + 50 = 86
 console.log(shifted)  // 86 (somewhere between 50 and 100)

Remember:

(max - min + 1) gives you the range size
Math.floor() converts to integer
+ min shifts to the correct starting point

Real-World Examples

Dice Roll (1-6)

⚡ javascript

function rollDice() {
  return Math.floor(Math.random() * 6) + 1
}
 console.log(rollDice())  // 1, 2, 3, 4, 5, or 6

Coin Flip

⚡ javascript

function flipCoin() {
  return Math.random() < 0.5 ? 'HEADS' : 'TAILS'
}
 console.log(flipCoin())  // HEADS or TAILS

Random Array Element

⚡ javascript

const colors = ['red', 'blue', 'green', 'yellow', 'purple']
 function randomColor() {
  const index = Math.floor(Math.random() * colors.length)
  return colors[index]
}
 console.log(randomColor())  // random color from array

Random Boolean (True/False)

⚡ javascript

function randomBoolean() {
  return Math.random() < 0.5
}
 console.log(randomBoolean())  // true or false
 // With different probability
function randomBoolean(probability = 0.5) {
  return Math.random() < probability
}
 console.log(randomBoolean(0.7))  // 70% chance of true

Challenge: Random Generator

Try these random generators and create your own custom range!

Roll a Die

Flip a Coin

Custom Range Generator

Min:

Max: