Indicates that there is no object , That is, there should be no value , Usage is as follows ：
As an argument to a function , The parameter representing the function is not an object ;
As the end of the prototype chain .
Indicates a lack of value , This is where there should be a value , But there is no definition , The situation is as follows ：
Variables are declared , But when there is no assignment , Is equal to undefined;
When you call a function , The parameter that should be provided is not provided , This parameter is equal to undefined;
Object has no assigned property , The value of this property is undefined;
When the function does not return a value , Default return undefined;