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How can I 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("Demo Text!"); } if ( typeof(display) === 'undefined') { document.write('undefined'); } else { document.write("Function is defined"); } </script> </body> </html>
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