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How to check if a variable exists in JavaScript?

To check if a variable exists in JavaScript, you need to check it against null as in the following code. Here, we’re checking the existence of variable myVar −

<html>
   <body>
      <script>
         var myVar = 20;
         if(myVar !== undefined && myVar !== null) {
            document.write("Variable exists");
         }
      </script>
   </body>
</html>