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How to check if a JavaScript function is defined?

To check if a JavaScript function is defined or not, checks it with “undefined”.

Example

You can try to run the following example to check for a function is defined or not in JavaScript −

<!DOCTYPE html>
<html>
   <body>
      <script>
         function display() {
            alert("Hello World!");
         }
         if ( typeof(display) === 'undefined')  {
            document.write('undefined');
         } else {
            document.write("Function is defined");
         }
      </script>
   </body>
</html>