Undefined and null

Both null and undefined can mean no, no, but there is a difference between them.

Undefined: When var declares a variable, but is uninitialized, the object is undefined.

For example:

var//undefined

While

var//true

The Null:null is a special value.

var NULL  //nullnull); // true

typeof (returns the data type of a value) under the action:

var Animal;console.log (typeof//undefinedconsole.log (typeof// undefined var NULL ; Console.log (typeof//objectconsole.log (typeofnull  //Object

Visible NULL

But if so:

NULL // true

Number (non-numeric converted to numeric) action

Console.log (number (null//0console.log (number (undefined));   NaN

！ Logical non-operational

Console.log (!  Null); // trueconsole.log (!undefined); // true

&& Logic and operations

varDog;console.log (Dog&&NULL);//NULLConsole.log (dog && undefined);//undefinedvarDig =NULL; Console.log (Dig &&NULL);//NULLConsole.log (dig && undefined);//NULLvarPig = "Pig"; Console.log (Pig&&NULL);//NULLConsole.log (pig && undefined);//undefined

Undefined and null