The Math object

Learn rounding with Math.round, Math.floor and Math.ceil.

The Math object

In JavaScript Math is a built-in object that holds useful constants and methods for numbers. You do not create a new Math; you just use it directly.

Three common rounding methods are:

Math.round(x) — rounds to the nearest integer.
Math.floor(x) — always rounds down to an integer.
Math.ceil(x) — always rounds up to an integer.

For example Math.round(4.5) gives 5, Math.floor(4.9) gives 4 and Math.ceil(4.1) gives 5.

Down or up?

Math.floor and Math.ceil always go the same direction; that is why they serve different real world jobs.

When you compute remaining stock in a shop you use Math.floor: 7.8 boxes means 7 full boxes. But when you count the pages needed you use Math.ceil: even 3.1 pages means 4 pages, because you cannot print half a page.

The job, not the number, picks the rounding direction.

Exercise Round the score down and up

Declare const score = 7.8;. Then declare down with Math.floor(score) and up with Math.ceil(score). Log both with console.log.

Show example solution

const score = 7.8;
const down = Math.floor(score);
const up = Math.ceil(score);
console.log(down);
console.log(up);

Test yourself

What does Math.round(4.5) return?
```
console.log(Math.round(4.5));
```
- 4
- 4.5
- 5
- Throws an error
Show answer

5 — Correct. Math.round sends 0.5 and above upward; for 4.5 the result is 5.

Round 9.99 to the nearest integer

const rounded = Math.round(____);

Show answer

const rounded = Math.round(9.99);

Match each method to its job

Math.round

Nearest whole number
Math.floor

Down to whole number
Math.ceil

Up to whole number
Math.PI

The pi constant (~3.14)
What does Math.ceil(3.1) return?
```
console.log(Math.ceil(3.1));
```
- 3
- 3.1
- Throws an error
- 4
Show answer

4 — Correct. Math.ceil always goes up; for 3.1 the next whole number up is 4.

Next lesson: Math.random and Math.min/max

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