javascript - How to check 'undefined' value in jQuery

JQuery library was developed specifically to simplify and to unify certain JavaScript functionality.

However if you need to check a variable against undefined value, there is no need to invent any special method, since JavaScript has a typeof operator, which is simple, fast and cross-platform:

if (typeof value === "undefined") {     // ... } 

It returns a string indicating the type of the variable or other unevaluated operand. The main advantage of this method, compared to if (value === undefined) { ... }, is that typeof will never raise an exception in case if variable value does not exist.

In this case you can use a === undefined comparison: if(val === undefined)

This works because val always exists (it's a function argument).

If you wanted to test an arbitrary variable that is not an argument, i.e. might not be defined at all, you'd have to use if(typeof val === 'undefined') to avoid an exception in case val didn't exist.

Note that typeof always returns a string, and doesn't generate an error if the variable doesn't exist at all.

function A(val){   if(typeof(val)  === "undefined")      //do this   else    //do this } 

I know I am late to answer the function but jquery have a in build function to do this

if(jQuery.type(val) === "undefined"){     //Some code goes here } 

Refer jquery API document of jquery.type https://api.jquery.com/jQuery.type/ for the same.

You can use shorthand technique to check whether it is undefined or null

 function A(val)  {    if(val || "")     //do this  else  //do this  } 

hope this will help you