When a variable is declared without initialization, it’s assigned the undefined value. It’s not very useful on its own. A variable is undefined if it’s declared, but no value has been assigned to it. In contrast, null is assigned to a variable, and it represents no value.
console.log(null == null); // true
console.log(undefined == undefined); // true
console.log(null == undefined); // true, with type-conversion
console.log(null === undefined); // false, without type-conversion
console.log(0 == undefined); // false
console.log('' == undefined); // false
console.log(false == undefined); // false
