How do you check a variable's type?

typeof handles primitives, but it has two well-known holes, so it is not a complete answer on its own.

typeof "hi";        // "string"
typeof 42;          // "number"
typeof true;        // "boolean"
typeof undefined;   // "undefined"
typeof Symbol();    // "symbol"
typeof 10n;         // "bigint"
typeof (() => {});  // "function"

typeof null;        // "object"  ← a bug kept for compatibility
typeof [];          // "object"  ← arrays are objects

Filling the gaps

value === null;              // the only reliable null check
Array.isArray(value);        // the correct array check

Array.isArray also works across iframes, where instanceof Array fails because each frame has its own Array constructor.

Distinguishing built-in objects

typeof says "object" for a Date, a Map and a plain object alike. When you need to tell them apart:

const typeOf = (v) => Object.prototype.toString.call(v).slice(8, -1).toLowerCase();

typeOf(null);        // "null"
typeOf([]);          // "array"
typeOf(new Date());  // "date"
typeOf(new Map());   // "map"
typeOf(/x/);         // "regexp"

instanceof for your own classes

err instanceof ValidationError;   // true for subclasses too

It checks the prototype chain, so it is right for class hierarchies and wrong for primitives — "hi" instanceof String is false.

Two practical notes

typeof is the only operator that does not throw on an undeclared variable, which makes typeof maybeGlobal !== "undefined" a safe existence check — except inside a temporal dead zone, where it throws like anything else.

And NaN is a number: use Number.isNaN(v) to detect it, and Number.isFinite(v) to reject NaN and both infinities at once.