To set the 2nd parameter as undefined, just set it to the following −
displayFunc(undefined, "Amit");
Example
Here’s an example displaying the second parameter successfully when the first is set as “undefined”
Live Demo
<!DOCTYPE html> <html> <body> <script> function displayFunc (p1, p2) { document.write("Parameter 1 = "+p1); document.write("<br>Parameter 2 = "+p2); } displayFunc(undefined, "Amit"); </script> </body> </html>