Why are preload link not working for JSON requests ?
The JavaScript on my website loads several JSON to initialize itself.
I would like to preload them so, when the JavaScript will launch an Ajax request on it, they will be loaded instantaneously.
A new link tag exists for that.
I tried to use it to load a JSON like that :
<link rel="preload" href="/test.json">
However, Chrome seems to load it twice and present a warning in the console :
The resources test.json was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.
So it seems that preload doesn’t work for JSON. Indeed, I haven’t found reference to JSON in the specification.
Is that correct or am I doing it wrong ?
Answer
According to https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content , you have to add as="fetch" for JSON files.
So your code becomes
<link rel="preload" href="/test.json" as="fetch">
It's supported by all modern browsers and you get a warning message if this resource is not used within a few seconds because it is counterproductive to "preload" it in a such case (delay, double load etc.)
It's different from <link rel="prefetch" ...> which is to anticipate future navigation and not supported widely.
A Chrome illustrated article about this: https://medium.com/reloading/preload-prefetch-and-priorities-in-chrome-776165961bbf