What is the process by which Single Page Applications manage the Not Modified 304 response?

Imagine a scenario where a Single Page Application (SPA) built using angular or vuejs loads 3 components on a page, with each component making requests to different backend APIs.

Now, if a user decides to refresh the page, those same 3 API calls are triggered again. However, this time the backend responds with a 304 status code for each request.

Since SPA components are essentially just JavaScript code, they expect data to be returned from the backend with every call. But when a 304 status code is received, it means there's no new data. Surprisingly, everything seems to work fine...

It appears like there's some sort of magic happening behind the scenes that I can't quite grasp. Could someone shed light on this mystery for me?

Answer №1

In the event that the client includes an If-Modified-Since or If-None-Match request header, the server will send back a 304 response with no content. If these headers are not included, the server will always return the full content. In this scenario, there are two possible outcomes:

The Javascript code itself could be responsible for adding these headers, indicating that it still contains the data in order to permit empty responses. This data may be stored in local storage by the Javascript code.
Alternatively, the browser might automatically include these headers if it holds an implicit cache of the data. If a 304 response is received from the server, the browser could then transparently provide the cached data to fulfill the Javascript request. While it is uncertain whether browsers actually operate in this manner, it remains a possibility.

It is more probable that the Javascript is managing a local cache, potentially utilizing a web worker for this purpose.

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