Undefined and null

 At first glance it may appear that null and undefined are basically the same, however there are subtle but

important differences.

undefined is the absence of a value in the compiler, because where it should be a value, there hasn't been put one,

like the case of an unassigned variable.

undefined is a global value that represents the absence of an assigned value.

typeof undefined === 'undefined'

null is an object that indicates that a variable has been explicitly assigned "no value".

typeof null === 'object'

Setting a variable to undefined means the variable effectively does not exist. Some processes, such as JSON

serialization, may strip undefined properties from objects. In contrast, null properties indicate will be preserved so

you can explicitly convey the concept of an "empty" property.

The following evaluate to undefined:

A variable when it is declared but not assigned a value (i.e. defined)

let foo;

console.log('is undefined?', foo === undefined);

// is undefined? true

Accessing the value of a property that doesn't exist

let foo = { a: 'a' };

console.log('is undefined?', foo.b === undefined);

// is undefined? true

The return value of a function that doesn't return a value

function foo() { return; }

console.log('is undefined?', foo() === undefined);

// is undefined? true

The value of a function argument that is declared but has been omitted from the function call

function foo(param) {

 console.log('is undefined?', param === undefined);

}

foo('a');

foo();

// is undefined? false

// is undefined? true

undefined is also a property of the global window object.

// Only in browsers

console.log(window.undefined); // undefined

window.hasOwnProperty('undefined'); // true 

Version < 5

Before ECMAScript 5 you could actually change the value of the window.undefined property to any other value

potentially breaking everything.