How to Generate Random Number in JavaScript: Simple Guide

Use Math.random() to generate a random decimal number between 0 (inclusive) and 1 (exclusive). To get a random number in a specific range, multiply the result and adjust with addition, then use Math.floor() or Math.round() to get whole numbers.

Syntax

The basic syntax to generate a random decimal number is Math.random(). It returns a number from 0 up to but not including 1.

To get a random integer in a range, use: Math.floor(Math.random() * (max - min + 1)) + min.

• Math.random(): generates a decimal between 0 and 1.
• Math.floor(): rounds down to the nearest whole number.
• max: the highest number you want.
• min: the lowest number you want.

javascript

Math.random()

// To get a random integer between min and max (inclusive):
Math.floor(Math.random() * (max - min + 1)) + min

Example

This example shows how to generate a random integer between 1 and 10 and print it.

javascript

const min = 1;
const max = 10;
const randomNumber = Math.floor(Math.random() * (max - min + 1)) + min;
console.log(randomNumber);

Output

7

Common Pitfalls

One common mistake is forgetting to add 1 when calculating the range, which causes the maximum number to never appear.

Another is using Math.round() which can bias the results unevenly.

javascript

// Wrong: max number never appears
const wrongRandom = Math.floor(Math.random() * (max - min)) + min;

// Right: includes max number
const rightRandom = Math.floor(Math.random() * (max - min + 1)) + min;

Quick Reference

Purpose	Code Snippet
Random decimal 0 to 1	Math.random()
Random integer 0 to max	Math.floor(Math.random() * (max + 1))
Random integer min to max	Math.floor(Math.random() * (max - min + 1)) + min

Key Takeaways

Use Math.random() to get a decimal between 0 and 1.

Multiply and add to scale the random number to your desired range.

Use Math.floor() to get whole numbers without bias.

Always add 1 to the range calculation to include the max number.

Avoid Math.round() for random integers to prevent uneven distribution.