What are the different data types in JavaScript?

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JavaScript has eight data types divided into two categories: seven primitive types and one reference type. Primitives are immutable values stored directly in the variable, while the single non-primitive type— Object —is stored by reference. The seven primitive types String — textual data written in single quotes, double quotes, or backticks. Example: "hello" , `template ${name}` . Number — both integers and floating-point values, including Infinity , -Infinity , and NaN . JavaScript has only one numeric type backed by 64-bit IEEE 754 doubles. BigInt — arbitrary-precision integers for values beyond Number.MAX_SAFE_INTEGER (253-1). Written with an n suffix: 9007199254740993n . Boolean — true or false . Undefined — the default value of a declared but unassigned variable. Its only value is undefined . Null — an intentional absence of any value. Developers assign it explicitly to signal "nothing here". Symbol — a unique, immutable identifier commonly used as non-colliding object keys. Symbol("id") !== Symbol("id") . The reference type Object covers every non-primitive value: plain objects, arrays, functions, dates, regexes, maps, sets, and class instances. Objects are mutable and compared by reference, not by value. Checking types Use typeof for primitives and Array.isArray() or instanceof for objects. One historical quirk to remember: typeof null === "object" , which is a long-standing bug preserved for backwards compatibility. typeof "hi" // "string" typeof 42 // "number" typeof 10n // "bigint" typeof true // "boolean" typeof undefined // "undefined" typeof null // "object" <-- the famous quirk typeof Symbol() // "symbol" typeof {} // "object" typeof [] // "object" typeof function(){}// "function" Primitive vs reference semantics Primitives are copied by value, so assignment creates an independent copy. Objects are copied by reference, so two variables can point to the same underlying data. let a = 5; let b = a; // b gets its own copy b = 10; // a is still 5 let x = { n: 5 }; let y = x; // y points to the same object y.n = 10; // x.n is now 10 Name all 7 primitives + object. The typeof null === 'object' bug is the #1 JS gotcha question. Know the proper type-checking methods: Array.isArray, === null, Number.isNaN.