Built-in Math functions

A function is like a small machine: you give it a value in brackets, called an argument, it does a job and gives back a result, called the return value. JavaScript has many ready-made functions you can use without writing them yourself.

For example, Math.round rounds a number to the nearest whole number:

JavaScript

console.log(Math.round(9.6));
console.log(Math.round(9.2));
console.log(Math.round(10.5));

Console

10
9
11

Function	What it does	Example	Result
Math.round	rounds to the nearest whole number	Math.round(10.5)	11
Math.floor	drops the fraction (rounds down)	Math.floor(10.5)	10
Math.max	the largest value	Math.max(5, 10, 50)	50
Math.min	the smallest value	Math.min(5, 10, 50)	5

Math.floor is useful when only the whole part matters. Math.max and Math.min take any number of arguments separated by commas, and they can be variables:

JavaScript

console.log(Math.floor(7.8));
let ali = 78;
let sara = 92;
let omar = 85;
console.log(Math.max(ali, sara, omar));
console.log(Math.min(ali, sara, omar));

Console

7
92
78

Write Math with a capital M, then a dot, then the function name. math.round or Math.Round gives an error.

Random numbers

Math.random() takes no argument and returns a random number from 0 up to (not including) 1, different every run. To get a whole number from 0 to 9, multiply by 10, then drop the fraction with Math.floor. Run it a few times:

JavaScript

console.log(Math.random());
console.log(Math.floor(Math.random() * 10));

Console

0.000054784119180202595
9

You can also use a random number in a condition, like tossing a coin:

JavaScript

if (Math.random() >= 0.5) {
  console.log("Lucky today");
} else {
  console.log("Unlucky today");
}

Console

Unlucky today

Practice

1Write the programCurriculum practice

Round 9.7 with Math.round and print the result.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

Console

Press Run to see the result.

2Write the programCurriculum practice

Print 5.5 rounded with Math.round, then 8.8 with its fraction dropped using Math.floor.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

Console

Press Run to see the result.

3Write the programCurriculum practice

Print the larger of 10 and 20 with Math.max, then the smaller with Math.min.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

Console

Press Run to see the result.

4Fill in the blankCurriculum practice

Complete the line so it prints a random whole number from 0 to 9: multiply Math.random() by 10, then drop the fraction.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

console.log(___);

Console

Press Run to see the result.

5Fill in the blankCurriculum practice

Complete the condition: if the random number is greater than or equal to 0.5, print Excellent luck, otherwise Bad luck.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

if (___) {
  console.log("Excellent luck");
} else {
  console.log("Bad luck");
}

Console

Press Run to see the result.

6Write the programExtra practice

A student scored 78, 85 and 91 in three tests. Work out the average (total ÷ 3), round it with Math.round, then print Average: 85.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

Console

Press Run to see the result.

7Write the programExtra practice

A school trip has 47 students and a minibus holds 12. Use Math.floor to print how many minibuses are completely full: Full buses: 3.

Hint

47 ÷ 12 = 3.9..., then drop the fraction.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

let students = 47;
let seats = 12;

Console

Press Run to see the result.

Key points

A function takes an argument in brackets and returns a value.
Math.round rounds, Math.floor drops the fraction, Math.max / Math.min give the largest and smallest value.
Math.floor(Math.random() * 10) is a random whole number from 0 to 9.