JS BASICSChapter 1 · JS Basics

Numbers and Math

JavaScript has one number type for whole numbers and decimals alike. Use +, -, * and / for arithmetic, % for the remainder and ** for powers. The built-in Math object has the helpers: Math.round, Math.floor, Math.max, Math.random and more. One surprise: 0.1 + 0.2 is not exactly 0.3, because decimals are stored in binary.

Worked example

How it reads

/ keeps the decimals; Math.floor rounds down to a whole number
% gives the remainder, so 17 % 5 is 2
.toFixed(2) turns a number into text with two decimal places

Common mistakes

"5" + 3 is "53": + joins text whenever either side is a string.
0.1 + 0.2 === 0.3 is false. Round before comparing decimals.

Cloud tip: n % 2 === 0 checks whether a number is even.