Distinguish undefined, null, and Nan in Javascript

Distinguish underfined, null, and Nan

First, let's look at the running result of the following statement:

 
1: VaRA;
 

 
2:Alert ();// Display "undefined"
 

 
3:Alert (TypeofA );// Display "undefined"
 

 
4:Alert (B); // display"Undefined"
 

The value and type of a are both 'underfined'

 
1: VaRA =Null;
 

 
2:Alert ();// Display "null"
 

3:Alert (TypeofA); // display"Object"
 

The preceding description is a null variable with a null value and an object type.

 
1: VaRB;
 

 
2:Alert (Null= B );// Display "true"
 

 
3:Alert (Null= Undefined); // display"True"
 

The preceding description shows that undefined and null are equal.

 
1: VaRA1;// The values and types are both 'underfined'
 

2: VaRA2 =Null;// The value is null and the type is object.
 

 
3:Alert (a1 = a2); // display"True"
 

The preceding two examples show that null and underfind are very similar. At least undefined and null are equal.

 
1:Alert (100 +Null);// Display 100"
 

 
2:Alert (100 + undefined); // display"Nan" 
 

The preceding description shows that null and underfind are similar, but there are still residential areas!

Underfined: Unknown variable name or unassigned variable.
Null: Special object
Nan: Special Number

1:Alert (Typeof(Undefined ));// Display 'undefined'
 

 
2:Alert (Typeof(Null));// Display 'object'
 

 
3:Alert (Typeof(""));// Display 'string'
 

 
4:Alert (Typeof(0 ));// Display 'number'
 

 
5:Alert (Typeof(False));// Display 'boolean'
 

6: 
 

 
7: VaRA7 = Nan;
 

 
8: VaRA8 = undefined;
 

 
9: 
 

 
10:Alert (TypeofA7 );// Display "Number"
 

 
11:Alert (TypeofA8); // display"Undefined"
 

The preceding description shows that Nan is a special number, which is not equal to null or undefined.

Function parameter judgment

Let's take a look at the following example:

 
1:Test =Function()
 

2:{
 

 
3:Alert (TypeofA );
 

 
4:}
 

 
5:
 

 
6:Test ();// If no parameter is set, the running result is "underfined"
 

 
7:Test (Null); // An empty parameter is passed and the running result is"Object"If you want to use a. Length and so on, an error is reported because the object is empty!
 

The correct parameter check is:

 
1:Test =Function(V)
 

 
2:{
 

3: If(V! =Null&&TypeofV! ='Undefined')
 

 
4:{
 

 
5: // Use the attributes of V, such as V. Length and V. Property.
 

 
6: // If (V. offsetx! = Undefined)
 

 
7: // If (value = undefined)
 

 
8:}
 

 
9:}
 

Of course, you can also use if (v) to simplify the encoding. You can check whether V is not set to or if V is null. However, note that if a Boolean of true or false is input, if (v) means if (V = true ),CodeThe logic may not be the result you want!

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