null and undefined The difference between

null: Represents an empty object
undefined: Represents that the variable is not assigned

summary ：

1. When a variable is defined but no value is assigned , Output undefined
2. The variable can be specified as null, But not assigned to undefined
3. When the variable is set to null when , The type is object

Example ：

️： When a variable is defined but not assigned a value , Output undefined.

	var a;
    console.log(a)

output：undefined

If you don't create a variable, use it , Output a is not defined

    console.log(a)

output：Uncaught ReferenceError: a is not defined;

️： When assigning a value to a variable ：

It can be assigned to null,
But it cannot be assigned to undefined

    var a = undefind;
    console.log(a)

output：undefind is not defined

    var a = null;
    console.log(a)

output：null

When the variable is set to null when , The type is object

	var a = null;
    console.log(typeof a)

output：object