how to check if string is null or empty using jQuery or javascript?

You can create a function which consider if value is not null or undefined

function CheckNullUndefined(value) {
  return typeof value == 'string' && !value.trim() || typeof value == 'undefined' || value === null;
}

It can be used as below

CheckNullUndefined(undefined); //returns true
CheckNullUndefined(null); //returns  true
CheckNullUndefined(''); //returns  true
CheckNullUndefined('foo'); //returns  false
CheckNullUndefined(1); //returns  false
CheckNullUndefined(0); //returns  false

OR

Simply call it like

if (!value.trim()) {
    // is empty or whitespace
}

But, preferred method should be the first one as it checks whether to use .trim or not, checks null and undefined value perfectly.

OR

Simply use this

if(value !== null && value !== '')
{
   //do something
}

Above will also do type equality check.

You can also simply do this to check if string is null or empty as below

if (a == null || a=='')
{
    //a is not null and empty
}

Thanks

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You can also simply use Dual NOT (!!) to check if string is empty or null

if (!!str) {
    // if not null or empty code here
}

OR 

you can also create your own function

function checkIfEmptyNull(e) {
  switch (e) {
    case "":
    case 0:
    case "0":
    case null:
    case false:
    case undefined:
      return true;
    default:
      return false;
  }
}

checkIfEmptyNull(null) // true
checkIfEmptyNull(0) // true
checkIfEmptyNull(7) // false
checkIfEmptyNull("") // true

thanks