Arithmetic and assignment operators

Arithmetic operators

Your first calculator lived in a classroom. JavaScript's lives in every app you use: prices, scoreboards, progress bars. Let's start there.

They are used to do calculations with numbers. They are the same ones you use in math, with a few additions specific to programming.

The remainder (%) is not the division

The % operator returns what is left over from an integer division. Think about sharing out candy: if you have 7 pieces of candy between 2 children, each one gets 3 and 1 is left over. That 1 is 7 % 2.

console.log(7 % 2); // 1  (1 left over)
console.log(8 % 2); // 0  (even: nothing left over)
console.log(9 % 3); // 0

That is why number % 2 === 0 is the classic way to tell whether a number is even.

Assignment operators

Once you have a variable, you can update it with shortcuts:

let points = 10;
points += 5;  // equivalent to: points = points + 5  → 15
points -= 3;  // → 12
points *= 2;  // → 24

Increment and decrement

++ adds 1 and -- subtracts 1. They are very common in loops:

let counter = 0;
counter++; // now it equals 1
counter++; // now it equals 2

With this you can already move numbers around at will. In the next lesson we stop calculating and start comparing: the first step toward making your code decide.

Examples

Basic calculations

const a = 17;
const b = 5;
console.log("sum:", a + b);
console.log("remainder:", a % b);
console.log("power:", b ** 2);

Assignment shortcuts

let balance = 100;
balance += 50;  // deposit
balance -= 30;  // expense
console.log("final balance:", balance);