How can I make my fetch requests include "Last-Modified" headers automatically?

Question

I'm using fetch api (https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API)

I'm getting a response with "Last-Modified" header (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified)

When I run the fetch a second time, it does not include the "Last-Modified" header in the request, and therefor fails at caching.

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I have read the following on caching and fetch (https://developer.mozilla.org/en-US/docs/Web/API/Request/cache) but it works with ETags.

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How can I make fetch include the "Last-Modified" header?

Answer 1

Last-Modified is a response header, not a request header.

The corresponding request header is If-Modified-Since. That's what you should expect to see in the request if you want caching.