3. Values and types: primitives, coercion, == vs ===

JavaScript has exactly 8 types: 7 primitives (undefined, null, boolean, number, bigint, string, symbol) plus object (which includes arrays, functions, dates, maps, sets — everything non-primitive).

typeof undefined   // "undefined"
typeof null        // "object"   <- famous historical bug, kept forever for compatibility
typeof 42          // "number"
typeof 42n         // "bigint"
typeof "hi"        // "string"
typeof true        // "boolean"
typeof Symbol()    // "symbol"
typeof {}          // "object"
typeof []          // "object"   <- arrays are objects; use Array.isArray()
typeof function(){} // "function" (a callable object)

number is a single IEEE-754 double-precision float type for everything — there's no separate int type, which is why 0.1 + 0.2 === 0.30000000000000004 and why integers beyond Number.MAX_SAFE_INTEGER (2^53 − 1) lose precision. BigInt (42n) exists specifically to represent arbitrary-precision integers when that matters (crypto, large IDs).

Coercion is JavaScript converting a value's type implicitly to make an operation work. == (loose equality) coerces; === (strict equality) never does — always use === unless you have a specific, commented reason not to.

"5" == 5        // true  — string coerced to number
null == undefined  // true — special-cased, both "nullish"
0 == false      // true  — boolean coerced to number
"" == 0         // true  — both coerce to 0
[] == false     // true  — array -> string "" -> number 0

"5" === 5       // false — no coercion, different types
null === undefined // false

Memorize the two == exceptions worth keeping (null == undefined is a genuinely useful "is this nullish" check) and otherwise ban == via ESLint's eqeqeq rule — the coercion table is not something anyone should rely on from memory in review.