Testing for undefined in JavaScript

I want to run a code only if the argume开发者_Go百科nt[0].recordCount is greater than zero or is NOT undefined. However, the code is ran when the argument[0].recordCound alert shows undefined.

if(arguments[0].recordCount > 0 && 
   arguments[0].recordCount !== 'undefined')
{ //if more than 0 records show in bar
                            
    document.getElementById('totalRecords').innerHTML = 
       arguments[0].recordCount + " Records";
}

How can I test for undefined here?

---

When using undefined as a string you need to do so with the typeof operator.

Also, you should be checking if it's defined before any other checks on the property.

if ( 'undefined' != typeof arguments[0].recordCount && arguments[0].recordCount > 0 )

---

undefined is a keyword a global variable with constant value, use it without quotes:

if(arguments[0].recordCount > 0 && arguments[0].recordCount !== undefined)

But actually it would be sufficient to test only the first condition:

if(arguments[0].recordCount > 0)

because if recordCount is larger than zero, it is defined anyway.

---

More common is to switch the conditions and test first whether it is defined, to avoid possible errors in the following tests (not sure if this is necessary here):

if(arguments[0].recordCount !== undefined && arguments[0].recordCount > 0)

---

To check for a variable to be not null and not undefined,

if(thatVariable) is enough though implicit conversion can cause problem for some cases where thatVariable is empty string, or a boolean, or number 0. If implicit conversion is not the case for our variable, the following would do,

if(arguments[0].recordCount && arguments[0].recordCount > 0)

But the following would be problematic,

if(arguments[0].recordCount !== undefined && arguments[0].recordCount > 0)

Consider,

var undefined = 'surprise' //possible since undefined is not a keyword
if(arguments[0].recordCount !== undefined && arguments[0].recordCount > 0)

Now this 'if' will break even though recordCount is undefined.

One more thing: if(a != null) will also check undefined due to implicit conversion. Hence if(a != null && a != undefined) is redundant