JS: undefined

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

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

[see JS: Value Types]

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

undefined is the value when:

1. Variable doesn't have a value (not initialized). [see JS: var Name Scope]
2. Accessing a non-existent property. [see JS: Get Property, Set Property]
3. Accessing array element with index out of bound, or non-existent element in a sparse array. [see JS: Understand JS Array]
4. When function argument is not given. [see JS: Function Parameters]
5. Return value of a function call when the function doesn't have return statement, or empty return statement.
6. other similar cases.

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

// accessing non-existent property returns undefined
const o = {"p":1};
console.log(
    o.b === undefined
); // true

// accessing out-of-bound array element returns undefined
const arr = ["a", "b"];
console.log(
    arr[9] === undefined
); // true

// When function argument is not given, value is undefined
function f(x) { return x; };
console.log(
    f() === undefined
); // true

// undefined is the return value of a function call when the function doesn't have return statement
function f() {};
console.log(
    f() === undefined
); // true

// undefined is the return value of a function with empty return statement
function f() {return;};
console.log(
    f() === undefined
); // true

[see JS: null]

ECMAScript 2015 §ECMAScript Data Types and Values#sec-ecmascript-language-types-undefined-type