JavaScript Numbers & Math
Math.randomgives a random decimal between 0 and 1.
It includes 0 but never reaches 1.
Everything else is built by scaling that range.
Example
javascript
const value = Math.random();
console.log(value >= 0 && value < 1);The output is true.
The value itself is different every time, so the check is what matters.
Multiply to widen the range.
Math.floor turns it into a whole number.
Add the minimum to shift the range where you want it.
Syntax
javascript
Math.floor(Math.random() * (max - min + 1)) + min;This gives a whole number from min to max, both included.
Multiply by 10 and floor the result.
Example
javascript
const n = Math.floor(Math.random() * 10);
console.log(n >= 0 && n <= 9);
console.log(Number.isInteger(n));Both are true on every run.
Add one to shift the range from 0-5 to 1-6.
Example
javascript
function roll() {
return Math.floor(Math.random() * 6) + 1;
}
const value = roll();
console.log(value >= 1 && value <= 6);Every roll lands inside the range.
The general formula covers both ends inclusively.
Example
javascript
function between(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
const value = between(10, 20);
console.log(value >= 10 && value <= 20);Leaving out the + 1 would never produce the maximum.
Use the array length as the range.
Example
javascript
const colours = ["red", "green", "blue"];
const pick = colours[Math.floor(Math.random() * colours.length)];
console.log(colours.includes(pick));The picked value is always one of the three.
Math.random is predictable enough that it must never be used for passwords or tokens.
Use crypto.getRandomValues for anything security-related.
Example
javascript
const value = Math.random();
console.log(typeof value === "number" && value < 1);It is fine for games, samples and shuffling.
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Math.random</title>
</head>
<body>
<h1>Math.random</h1>
<p id="out"></p>
<button id="roll">Roll again</button>
<script>
function between(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
function show() {
const colours = ["red", "green", "blue"];
document.getElementById("out").innerHTML =
"Dice: " + between(1, 6) +
"<br>Between 10 and 20: " + between(10, 20) +
"<br>Colour: " + colours[Math.floor(Math.random() * colours.length)];
}
show();
document.getElementById("roll").addEventListener("click", show);
</script>
</body>
</html>Run the above example in the Try It Editor.
Try rolling repeatedly:
Click the button several times and watch the values change.
Math.random gives a decimal from 0 up to but not including 1.Math.floor makes it a whole number.Every random value you need is built from this one function.
The + 1 in the range formula is the part people get wrong.
For security, reach for the crypto API instead.