How to check for “undefined” variable in JavaScript?

Javascript HTML Programming Scripts

With JavaScript, I want to find whether a variable is ‘undefined’. The following is my code, but I am not sure about it and it checks both undefined and null:

function isEmpty(age) {
   return (age === undefined || age == null) ? true : false;
}

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1 Answer

Samual Sam

Answered on 18th Jan, 2018

On getting the result of the following, you can find whether a variable is ‘undefined’. If the result is “false”, it means the variable is undefined.

Here, the variable results in “True”

Example

Live Demo

<html>
   <body>
   
      <script>
         var res = 10;

         if(res) {
            document.write("True");
         } else {
            document.write("False");
         }
      </script>
      
    </body>
</html>

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How to check for “undefined” variable in JavaScript?

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