TopolearnEvery bug about unexpected comparisons or 'undefined is not a function' starts with not knowing what type a value has.
JavaScript is dynamically typed: variables don't have types, values do. Every value is either an object or one of seven primitives:
string— text,"hi"number— 64-bit floating point,42,3.14,NaNbigint— arbitrary-size integers,10nboolean—true/falseundefined— "no value assigned"null— "intentionally empty"symbol— unique identifiers
Everything else — arrays, functions, dates, plain {} — is an object.
Primitives are immutable and compared by value: "a" === "a" is true. Objects are compared by reference: two separate {} are never equal, even with identical contents.
typeof tells you the type as a string. It has two famous quirks: typeof null is "object" (a bug kept for compatibility) and typeof of a function is "function". Use Array.isArray(x) for arrays and x === null for null.
When a value is used as a condition, it is converted to a boolean. Exactly these values are falsy: false, 0, -0, 0n, "", null, undefined and NaN. Everything else is truthy — including "0", "false", [] and {}.
Calling a method on a primitive, as in "abc".toUpperCase(), works because the engine temporarily wraps it in an object (String). You never need new String() yourself.
Example
console.log(typeof "hi"); // "string"
console.log(typeof 42); // "number"
console.log(typeof 42n); // "bigint"
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object" (historic quirk)
console.log(typeof [1, 2]); // "object"
console.log(typeof (() => {})); // "function"
console.log(Array.isArray([1])); // true
console.log({} === {}); // false: different objects
console.log(Boolean(""), Boolean("0"), Boolean([])); // false true trueRemember
- Seven primitives plus objects; primitives compare by value, objects by reference
- Falsy: false, 0, -0, 0n, "", null, undefined, NaN — everything else is truthy
typeof null === "object"; test arrays withArray.isArray
Watch out for
- Assuming an empty array or object is falsy —
if ([])runs its body.