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
