Distinguish between Undefined,null and Nan in JavaScript

differentiate between Underfined,null and Nan

Let's look at the result of the following statement:

1: var A;

2: //Show "undefined"

3: alert (typeof//Show "undefined" )

4: alert (b); Show "Undefined"

The values and types of a above are ' underfined '

1: var null;

2: //Show "null"

3: alert (typeof a); Show "Object"

The above description assigns a null variable, the value is null, the type is Object

1: var b;

2: alert (null//display "true"

3: alert (null = = undefined); Show "true"

The above description undefined and null are equal

1: var //value and type are ' underfined '

2: var NULL //value is NULL, type is Object

3: alert (a1 = = A2); Show "true"

The above two examples show that null and Underfind are very similar, at least undefined and null are equal.

1: NULL //Show "+"

2: alert (+ undefined); Show "NaN"

The above description of NULL and Underfind although very similar, but there are small differences, hey!

underfined: Unknown variable name, or non-assigned variable.
NULL: Special Object
NaN: Special number

1: alert (typeof(undefined)); //Show ' undefined '

2: alert (typeof(null)); //Show ' object '

3: alert (typeof(")"); //Display ' string '

4: alert (typeof(0)); //Show ' number '

5: alert (typeof(false)); //Show ' Boolean '

6:

7: var a7 = NaN;

8: var a8 = undefined;

9:

: alert (typeof//display "number"

One : alert (typeof A8); Show "Undefined"

The above indicates that Nan is a special number, and null and undefined are not equal.

The function parameter is not and is empty judgment

Let's take a look at the following example:

1: function (a)

2: {

3: alert (typeof a);

4: }

5:

6: //No parameters, the operation result is "underfined"

7: Test (null); Passed empty parameters, the result is "Object", if you want to use a.length, etc., will be an error, because the object is empty!

The correct parameter check is:

1: function (v)

2: {

3: if NULL typeof ' undefined ')

4: {

5: //Using properties of V, such as V.length, V.property

6:

7: //if (value = = undefined)

8: }

9: }

Of course, you can also use if (v) to simplify the encoding, you can check V is not given and V is null case, but note that if passed in a Boolean true or FALSE, with if (v) means if (v = = true), the code logic may not be the result you want!

Distinguish between Undefined,null and Nan in JavaScript