null vs undefined

Differences

1. undefined indicates that a variable has been declared but not been assigned to any value, including null.

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

   null is used to indicate that a variable doesn't have a value.

   let foo = null;
   console.log(foo); // null

2. undefined and null are considered to be different types.

   undefined is a type itself, whereas null is a special value of object.

   console.log(typeof undefined); // 'undefined'
   console.log(typeof null); // 'object'

   Since they are different types, here are the result of equality and identity operators when comparing them to each other:

   null == undefined; // true
   null === undefined; // false

Good to know

JSON.stringify omits undefined, but keeps null:

JSON.stringify({
    name: 'John',
    address: null,
    age: undefined,
});

// {"name":"John","address":null}