JavaScript: undefined

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

console.log(
  globalThis.hasOwnProperty("undefined"),
  globalThis["undefined"] === undefined,
);

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

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

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

undefined is the value when:

• Variable doesn't have a value (not initialized). [see let Declaration]
• Accessing a non-existent property. [see Get Property, 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.log(xx === undefined);

// accessing non-existent property return undefined

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

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

// When function argument is not given, value is undefined
function ff(x) {
  return x;
}

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

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

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

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

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