What is the difference between null and undefined?

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Both null and undefined represent "no value", but they carry different meanings and are produced in different situations. Treating them as interchangeable is a common source of bugs. undefined undefined means the engine did not assign a value. You see it for: Variables declared but not initialised. Function parameters that were not passed. Missing object properties. Functions with no explicit return . let x; // x === undefined function f(a) { return a; } f(); // undefined const obj = {}; obj.missing; // undefined null null is an intentional absence of value. Developers assign it to signal "this variable deliberately holds nothing". APIs often use it to indicate an empty slot or a cleared reference. let selected = null; // nothing selected yet document.getElementById("missing"); // null when not found Type and equality typeof undefined → "undefined" typeof null → "object" (a long-standing JavaScript quirk) null == undefined → true (they are loosely equal to each other and to nothing else) null === undefined → false Numeric coercion Number(null); // 0 Number(undefined); // NaN 1 + null; // 1 1 + undefined; // NaN Default parameters Default values kick in only when the argument is undefined , not when it is null . function greet(name = "Guest") { return name; } greet(); // "Guest" greet(undefined); // "Guest" greet(null); // null Nullish coalescing The ?? operator returns the right-hand side only when the left is null or undefined , which is safer than || when 0 , "" , or false are valid values. const count = value ?? 10; // uses 10 only if value is null/undefined When to use each Let the engine produce undefined naturally — rarely assign it yourself. Use null when you need to explicitly say "no value" and want downstream code to detect it. Check for either with value == null — the only universally accepted use of == . undefined = automatic/uninitialized, null = intentional absence. Know typeof null = 'object' (bug). The == null shorthand and ?. / ?? operators for null-safe access are practical modern tools.