JavaScript — Arithmetic

The seven operators

let a = 10, b = 3;

a + b    // 13    add
a - b    // 7     subtract
a * b    // 30    multiply
a / b    // 3.3333…  divide
a % b    // 1     remainder after division
a ** b   // 1000  power (10³)

% — the underrated one

Remainder answers "is it divisible?" instantly:

n % 2 === 0        // even?
i % 2              // 0=even row, 1=odd row (zebra tables!)
minutes % 60       // leftover minutes past full hours
items.length % 3   // how many overflow the last row of 3

++ and --

let lives = 3;
lives++;   // 4 — add one
lives--;   // 3 — remove one

Two flavors exist (++x vs x++ — returns new vs old value), but in modern code you'll almost always see them as standalone statements where they behave identically.

Order of operations

Math rules apply; parentheses win:

2 + 3 * 4       // 14
(2 + 3) * 4     // 20
2 ** 3 ** 2     // 512 — powers go right-to-left!

When expression order isn't obvious at a glance, add parentheses even if redundant — free documentation.

The floating-point surprise

0.1 + 0.2            // 0.30000000000000004  😱
0.1 + 0.2 === 0.3    // false

Computers store decimals in binary; tiny representation errors leak into decimal math. Every language with IEEE floats does this — JS just doesn't hide it.

Practical fixes:

(0.1 + 0.2).toFixed(2)          // "0.30" — display formatting
Math.round((0.1+0.2) * 100)/100 // 0.3   — rounding before comparing

Money? Store cents as integers. Always.

Numbers from text

"5" + 1      // "51"  ← plus means CONCATENATION when a string is present!
"5" - 1      // 4     ← minus forces number conversion 😵

That asymmetry is the most infamous JS gotcha. Convert deliberately:

Number("5") + 1    // 6
+"5" + 1           // 6 — unary plus shortcut
parseInt("42px")   // 42 — stops at first non-digit

Mini Practice

1. Compute all seven operators on 17 and 5; predict before running
2. Use % to print whether loop counters 1–15 are odd or even
3. Reproduce 0.1+0.2; fix display with toFixed and comparison with rounding
4. Explain "5"+1 vs "5"-1 in one comment line each
5. Build seconds→mm:ss converter using / and % together

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