how to create timer.js by JavaScript?

Answer 1

What have you tried so far? Have you looked at some Google search results, for instance? You can find many tutorials and examples there. so have a look at sololearn playground.
https://www.geeksforgeeks.org/javascript-timer/
https://www.educba.com/timer-in-javascript/
https://www.javatpoint.com/javascript-timer
https://www.geeksforgeeks.org/create-countdown-timer-using-javascript/
https://www.w3schools.com/howto/howto_js_countdown.asp

If you need help with your own code, please link it so we can check on it.

Please tag only "javascript", not "java".