Types & Operators

Wires are installed. Now: what's actually allowed to flow through them?

Every value has a type

JavaScript sorts every value into a type — what kind of thing it is, and what you're allowed to do with it. The three you'll use constantly:

let count = 12;           // number
let label = "porch lamp"; // string
let isOn = true;           // boolean

A number is just that — no quotes, and you can do math with it. A string is text, and it always needs quotes (single ' or double ", either works, just match the one you opened with) — 12 and "12" look similar but behave completely differently, as you're about to see. A boolean is one of exactly two values, true or false — a switch that's either on or off, nothing in between.

Curious what type a value actually is? typeof count answers that — try it, it hands back "number" as a string of its own.

Operators: combining and comparing values

+, -, *, and / do arithmetic on numbers, exactly like a calculator: count + 1, count * 2. But + has a second job — glue two strings together:

let greeting = "Hi, " + label;

That line builds one longer string out of two shorter ones.

To compare two values instead of combining them, JavaScript uses === — "is strictly equal to" — and !== for "is not equal to," plus <, >, <=, >= for ordering numbers. Each of these hands back a boolean: count === 12 is true, count === 13 is false.

Common mistake

+ doesn't know whether you mean math or gluing until it sees what's on both sides — and if either side is a string, it glues instead of adding: "5" + 5 is "55", a six-character string, not 10. Keep your numbers as actual numbers (no quotes) when you want math, and this trap mostly disappears.

One more trap, for later: JavaScript also has a looser ==, which converts types before comparing — it's the reason "5" == 5 claims to be true. === never does that silent conversion. Use ===, always, and you'll sidestep a genuinely famous class of JavaScript bugs.

Next: functions — where you'll finally wire up a piece of logic once and trigger it as many times as you like.

There's a thermostat in the lab to read first.