1. different meanings
undefined: when using var to declare variables but did not indicate it to be initialized, the value of this variable is undefined
null: null is a pointer to the object, it expressed its readiness to hold objects, not really saving value of the object,
If you define a variable to hold the object to prepare in the future, should the variable is initialized to null.
2. Different types
Uninitialized defined value is detected typeof "undefined" (character string),
And null values are detected typeof "object" (String)
console.log(typeOf undefined);//undefined console.log(typeOf null);//object
3. When the value is converted to a different
