## Background
Previously we introduced automatic caching for `fetch` based on certain
heuristics that were a bit tricky to grasp all scenarios. The scenarios
we would automatically cache were no dynamic data access before the
fetch call e.g. `headers()` or `cookies()`, the fetch call is inside of
a dynamic page e.g. `POST` method or `export const revalidate = 0` page
and the fetch is a non-`GET` request or has `Authorization` or `Cookie`
headers, or the fetch had `cache: 'no-store' | 'no-cache'` or
`revalidate: 0`.
## New Behavior
By default fetches will no longer automatically be cached. Instead they
need to be opted-in to caching via `export const fetchCache =
'default-cache' | 'force-cache',` `next: { revalidate: false or value >
0 }` or `cache: 'force-cache' | 'default-cache'`.
When the fetch call is automatically skipping the cache it won't impact
the page level ISR cacheability although if a fetch call manually
specifies `cache: 'no-store'` or `revalidate: 0` it will still bail from
the page being statically generated as it was before.
To achieve the previous behavior of automatic fetch caching all that
needs to be added is `export const fetchCache = 'default-cache'` in the
root layout(s) of your project.
Use dynamic import instead of require to load the incremental cache
handled, so when using ESM it will still work.
Updated the tests and merged them into new test suite, include 3 cases
of custom cache definition:
- CJS with `module.exports`
- CJS with `exports.default` with ESM mark
- ESM with `export default`
Closes NEXT-1924
Fixes#58509
JJ Kasper