JavaScript Math.floor()

Math.floor(x) rounds a number toward negative infinity and returns the greatest integer less than or equal to the input. With negative decimals, the result becomes more negative.

console.log(Math.floor(7.9));   // 7
console.log(Math.floor(-7.1));  // -8

Show Table of Contents ↓

Math.floor() syntax Top ↑

Math.floor(number)

Positive numbers Top ↑

console.log(Math.floor(11.257)); // 11
console.log(Math.floor(1.56));   // 1
console.log(Math.floor(4));      // 4

Negative numbers Top ↑

console.log(Math.floor(-3.564)); // -4
console.log(Math.floor(-7.164)); // -8

Invalid and missing values Top ↑

console.log(Math.floor());       // NaN
console.log(Math.floor("abc"));  // NaN

See NaN for invalid numerical results.

Math.floor(), Math.round(), Math.ceil() and Math.trunc() Top ↑

const value = -5.42;
console.log(Math.floor(value)); // -6
console.log(Math.round(value)); // -5
console.log(Math.ceil(value));  // -5
console.log(Math.trunc(value)); // -5

Math.trunc() removes the fractional part toward zero, so it differs from Math.floor() for negative values. See Math.ceil(), Math.round(), and toFixed().

Using Math.floor() for random integers Top ↑

const die = Math.floor(Math.random() * 6) + 1;
console.log(die); // 1 through 6

Infinity, negative zero and coercion Top ↑

Math.floor(Infinity) remains Infinity, while invalid numeric input produces NaN. A negative zero can also be preserved in some cases. For form input, convert and validate explicitly instead of depending on implicit coercion.

console.log(Math.floor(Infinity)); // Infinity
console.log(Math.floor("8.9")); // 8

Common Math.floor() mistakes Top ↑

Math.floor() does not merely remove decimal digits. For negative values it moves toward negative infinity, so Math.floor(-3.2) is -4, while Math.trunc(-3.2) is -3. Choose the operation that matches the rule you actually need.

Using Math.floor() for indexes and buckets Top ↑

const seconds = 367;
const minutes = Math.floor(seconds / 60);
console.log(minutes); // 6 complete minutes

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

About the author   Watch on YouTube

Connect: