JS: undefined

What is undefined

undefined is the value of the property key "undefined" of the Global Object.

console.assert((Object.hasOwn(globalThis, "undefined")) === true);

console.assert(globalThis["undefined"] === undefined);

undefined is a literal value. For example, you can write let x = undefined;

Type of undefined

Type of undefined is "undefined", and it is the only value of the type.

console.assert(typeof undefined === "undefined");

Purpose of undefined

undefined is the value when:

• Variable doesn't have a value (not initialized). 〔see let Declaration〕
• Accessing a non-existent property. 〔see Get Set Property〕
• Accessing array element with index out of bound, or non-existent element in a Sparse Array.
• When function argument is not given. 〔see Function Parameters〕
• Return value of a function call when the function doesn't have return statement, or empty return statement.
• other similar cases.

let xx;
console.assert(xx === undefined);

// accessing non-existent property return undefined

console.assert({ "kk": 3 }.kk === 3);
console.assert({ "kk": 3 }.xx === undefined);

// accessing out-of-bound array element returns undefined
console.assert([1, 2][9] === undefined);

// When function argument is not given, value is undefined

function ff(x) {
 return x;
}

console.assert(ff() === undefined);

// undefined is the return value of a function call when the function doesn't have return statement
function ff() {}

console.assert(ff() === undefined);

// undefined is the return value of a function with empty return statement
function ff() {
 return;
}

console.assert(ff() === undefined);