To declare optional function parameters in JavaScript, use the “default” arguments.
Example
You can try to run the following code to declare optional parameters −
<html> <body> <script> // default is set to 1 function inc(val1, inc = 1) { return val1 + inc; } document.write(inc(10,10)); document.write("<br>"); document.write(inc(10)); </script> </body> </html>