Differences between undefined and null in js

Data of the undefined type is used when no value is assigned to a declared variable and no declaration is made.
Before using js variables, you must declare them first.
If an undefined variable x is used, an error is returned.
How to determine whether a variable can be used:

Method 1:
If (typeof x = 'undefined ')
Var n = 100;
N = new Date ();
N = null;
Null indicates a clearly known value and is an empty object.

Method 2:

If (typeof (y )! = 'Undefined' & v! = Null) // You can also determine whether a variable is available.

Method 3:

Var v;

If (v)

{// V is converted to the Boolean Type and then judged.

Alert ('available ');

}

Else {

Alert ('unavailability ');

}

We seldom use the first and second methods, and usually use the third method.

In s, the use of undefined and null with = is equal, and the use of = is not equal.