Null and undefined

undefined

1. Statement , Assign to

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

2. A property of the object does not exist

let obj = {}
console.log(obj.a); // undefined

3. Missing parameters

function fn(a, b) {
    console.log(a, b); // 4 undefined
}

fn(4);

4. Method

function abcd() {
    console.log("111")
}

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

null

1. Manual memory release
2. As an argument to a function ( This parameter is not an object ）
3. Top of prototype chain

let obj = {};
obj1 = null;