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JavaScript Book

Undefined and Null

Both undefined and null represent “no value”, but they are used in slightly different contexts.

  • undefined means a variable has been declared but not assigned a value. Also the default return value of functions that don’t return anything.

    let x;
    console.log(x); // undefined
    

    function foo() {}
    console.log(foo()); // undefined
    null is an assignment value that represents "no object" or "empty". It must be explicitly set.
let y = null;
console.log(y); // null

Comparison:

console.log(undefined == null); // true (loose equality)
console.log(undefined === null); // false (strict equality)

In practice, use null when you want to explicitly indicate that a variable has no value, and let undefined represent unintentional absence.