JavaScript - why is null an object and what's the difference between null and undefined?

1. undefined is a primitive value automatically assigned to variables that have just been declared. It is where no notion of the variable exists, it has no type and it's never been referenced before in that scope,

2. null represents the intentional absence of any object value (e.g the variable is known to exist but it is not known what the value is).

Note:

null is not the same as false or "" values, even if it's treated as falsy for boolean operations:

// ONLINE-RUNNER:browser;

console.log(false == '');   // true
console.log(false == null); // false
console.log('' == null);    // false

References

1. undefined - JavaScript | MDN
2. null - JavaScript | MDN