Resource not found ERR:404 in express

I am making a simple express app,using react (with cdn).But my browser cannot get (resource not found ERR:404) the app.js file in which I have written my react code although I am rendering this file in an another file product.ejs using getelementbyid,and this(product ) file is rendering fine in browser.

I have not written route for app.js file as i am rendering it in product.ejs file

What can be the reason for this problem ?

This is first time I am asking question on stackoverflow .So, please respond if I can ask it in a good other way ✌️