Numbers and Operators

The operators

console.log(10 + 3);  // 13   add
console.log(10 - 3);  // 7    subtract
console.log(10 * 3);  // 30   multiply
console.log(10 / 4);  // 2.5  divide
console.log(10 % 3);  // 1    remainder ("what's left over")
console.log(2 ** 3);  // 8    power (2 × 2 × 2)

Order matters, just like in maths class: brackets first, then **, then * / %, then + -. Operators with the same rank run left to right. When in doubt, add brackets — they cost nothing and make your intention clear.

const price = 10 + 5 * 2;
console.log(price); // 20, not 30

const price = (10 + 5) * 2;
console.log(price); // 30

Remainder: the most underrated operator

% gives what's left after dividing. It answers questions like "is this number even?" (n % 2 is 0) and "125 minutes is how many hours and minutes?" Pair it with Math.floor, which chops off the decimals:

const minutes = 125;
const hours = Math.floor(minutes / 60); // 2
const rest = minutes % 60;              // 5
console.log(hours + "h " + rest + "m"); // 2h 5m

Changing a number in place

let points = 10;
points += 5;  // same as points = points + 5   → 15
points -= 3;  // same as points = points - 3   → 12
points *= 2;  // same as points = points * 2   → 24
points++;     // add exactly 1                 → 25
points--;     // subtract exactly 1            → 24

NaN: "Not a Number"

When maths makes no sense, JavaScript doesn't crash — it gives NaN. For example Number("RM12") is NaN because RM12 isn't a number, and "abc" * 2 is NaN too. Any maths with NaN gives NaN, so one bad value can quietly spoil a whole calculation. Also watch out for + with text: "12.50" + 5 joins them into "12.505". Convert text to a number first with Number(...).

The 0.1 + 0.2 surprise

console.log(0.1 + 0.2);            // 0.30000000000000004  (!)
console.log((0.1 + 0.2).toFixed(2)); // "0.30"  (a string, for display)
console.log(Math.round(2.5));       // 3
console.log(Math.round(1234.5678 * 100) / 100); // 1234.57

Computers store decimals in binary, and numbers like 0.1 can't be stored exactly — the same way 1/3 can't be written exactly as 0.333… This isn't a JavaScript bug; most programming languages do it. For money, two good habits: show amounts with toFixed(2) (it gives back a string like "25.10"), and calculate in whole sen when precision matters (Math.round(ringgit * 100)).