How to Generate Random Numbers in JavaScript

Use Math.random() when you need random values for games, UI variations, test data, IDs, or quick utilities. The key is converting the decimal result into the exact range you need.

What you’ll build or solve

You’ll learn how to generate random numbers in JavaScript with Math.random(). You’ll also know how to create whole numbers, ranges, and safe min-to-max values.

When this approach works best

This approach is the right choice when approximate randomness is enough for UI, games, or non-secure utilities.

Common real-world scenarios include:

• Dice rolls
• Random quotes
• A/B UI variations
• Shuffle helpers
• Mock data

This is a bad idea for passwords, tokens, or security-sensitive randomness. Use the Web Crypto API instead.

Prerequisites

You only need:

• A JavaScript file or browser console
• Basic math knowledge

Step-by-step instructions

Step 1: Use Math.random() and scale the result

Math.random() returns a decimal from 0 up to, but not including, 1.

const randomDecimal = Math.random();
console.log(randomDecimal);

To generate a whole number from 0 to 9, multiply and floor it.

const randomNumber = Math.floor(Math.random() * 10);
console.log(randomNumber);

To generate a min-to-max range, use this formula.

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

What to look for:

• Math.random() returns decimals
• Multiply to scale the range
• Use Math.floor() for whole numbers
• Add min for custom ranges
• Not suitable for secure randomness

Examples you can copy

Dice roll

const dice = Math.floor(Math.random() * 6) + 1;

Random index

const index = Math.floor(Math.random() * items.length);

Random range

const age = randomBetween(18, 65);

Common mistakes and how to fix them

Mistake 1: Forgetting Math.floor()

What the reader might do:

const roll = Math.random() * 6;

Why it breaks: this returns decimals like 4.28.

Corrected approach:

const roll = Math.floor(Math.random() * 6);

Mistake 2: Missing the inclusive max adjustment

What the reader might do:

Math.floor(Math.random() * (max - min)) + min;

Why it breaks: the max value can never appear.

Corrected approach:

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

Mistake 3: Using it for passwords or tokens

What the reader might do:

const token = Math.random().toString(36);

Why it breaks: Math.random() is not cryptographically secure.

Corrected approach:

Use the Web Crypto API for secure randomness.

Troubleshooting

If decimals appear, add Math.floor().

If the max value never shows, add + 1 inside the range math.

If the range starts too high, verify the + min part.

If security matters, avoid Math.random().

Quick recap

• Use Math.random() for decimals
• Multiply to scale the range
• Use Math.floor() for integers
• Add min for custom ranges
• Avoid it for secure tokens