TL;DR

extract some value from the file that changes with its content --like content's md5, or modified timestamp-- and append it to the file name

for example, let's say "index.js" and its md5 is "30c2234" (shortened for readability)

    <script src="file-30c2234.js">

later when you modify the file, the hash change, and clients start to request the new one.

this allows to set cache's evict time in doomsday