Math.floor() / ceil() / round() / trunc()

Methods for converting a number to an integer by handling its fractional part. There are four types: floor (round down), ceiling (round up), rounding, and truncation.

Syntax

// Rounds down to the nearest integer.
var num = Math.floor(number);

// Rounds up to the nearest integer.
var num = Math.ceil(number);

// Rounds to the nearest integer.
var num = Math.round(number);

// Removes the fractional part without rounding.
var num = Math.trunc(number);

Method List

Method	Description
Math.floor(number)	Returns the largest integer less than or equal to the given number. For positive numbers, it rounds down; for negative numbers, it moves to a smaller integer.
Math.ceil(number)	Returns the smallest integer greater than or equal to the given number. For positive numbers, it rounds up; for negative numbers, it moves to a larger integer.
Math.round(number)	Returns the integer nearest to the given number. If the fractional part is exactly 0.5, it rounds up.
Math.trunc(number)	Returns the integer part of a number by simply removing the fractional digits. For positive numbers, the result is the same as floor; for negative numbers, the same as ceil.

Sample Code

// Comparison with positive numbers
console.log(Math.floor(3.7)); // Outputs '3'. Rounds down.
console.log(Math.ceil(3.2)); // Outputs '4'. Rounds up.
console.log(Math.round(3.5)); // Outputs '4'. Rounds to nearest.
console.log(Math.trunc(3.7)); // Outputs '3'. Removes the fractional part.

// Behavior differs with negative numbers.
console.log(Math.floor(-3.2)); // Outputs '-4'. Moves to the smaller integer.
console.log(Math.ceil(-3.7)); // Outputs '-3'. Moves to the larger integer.
console.log(Math.round(-3.5)); // Outputs '-3'.
console.log(Math.trunc(-3.7)); // Outputs '-3'. Only the fractional part is removed.

// Practical example: generate a random integer from 0 to 9
var random = Math.floor(Math.random() * 10);
console.log(random); // Outputs one of the integers from 0 to 9.

// Practical example: calculate the number of pages
var totalItems = 23;
var itemsPerPage = 10;
var totalPages = Math.ceil(totalItems / itemsPerPage);
console.log(totalPages); // Outputs '3'. Displaying 23 items at 10 per page requires 3 pages.

Overview

All four methods convert a number to an integer, but they differ in how they handle the fractional part. The most commonly used is Math.floor(), and the pattern of combining it with Math.random() to generate a random integer is a classic technique.

When working with positive numbers only, Math.floor() and Math.trunc() produce the same result. However, with negative numbers they differ: Math.floor(-3.2) returns -4, while Math.trunc(-3.2) returns -3. When you simply want to strip the decimal part, Math.trunc() is more intuitive.

Math.ceil() is commonly used for pagination calculations. To display 23 items at 10 per page, Math.ceil(23 / 10) gives you the required number of pages: 3.