CODEWITHSUNDEEP
javascripthtmlxmlhttprequestfetchpreload
Ayush
Ayush

Reputation: 103

Can I add custom headers while trying to preload XHR data?

I am firing an XHR from JS code. This render-blocking XHR is fired on every page load, so I want to fetch it early in page's lifecycle, in parallel with JS and CSS resources. Inside my index.html, I'll add the following code:

<head>
  <link rel="stylesheet" href="style.css">
  <link rel="preload" as="fetch" href="/xhr-to-be-fetched-early">
</head>

The XHR request needs some custom headers such as authorization & Accept. Is there any way to add these headers to link tag, either inside HTML itself or via JS? Or is it impossible to cache such requests?

Upvotes: 9

Views: 1394

Answers (3)

divinelemon
divinelemon

Reputation: 2097

You cannot truly add headers to a <link> tag, but you can stop the page from loading anything until the resource is fetched.

See something like this:

const promiseOfSomeData = fetch("URL.com", { 
    // This is where you set the headers
    headers: {
        'X-My-Cool-Header': 'here'
    }
}).then(data => {
    return data;
});
window.onload = async () => {
    let someData = await promiseOfSomeJsonData;
    console.log(someData)
};

How can I make "onload" waiting for "fetch" complete?

Upvotes: 9

Kashan Ahmad
Kashan Ahmad

Reputation: 175

A unique situation, although, I have an idea popping in my mind that says how about you replace your preload link with a small JS snippet that requests the data using XHR, or fetch, or whatever method you prefer. Replace this:

<link rel="preload" as="fetch" href="/xhr-to-be-fetched-early">

with this:

<script async>
    fetch('target-url', {
        headers: {
            'Content-Type': 'application/json',
            //add headers here
        }
    })
    .then(response => response.json())
    .then(response => console.log("Preloaded", response))
</script>

Hope it helps, see more about fetch here.

async attribute in the script tag isn't necessary but it'll help reduce the load times if the data is kinda large.

Upvotes: 1

oieduardorabelo
oieduardorabelo

Reputation: 2985

you can't add custom headers in the link request; when you use as to specify the type of content, browsers will automatically apply the correct headers based on the content type, but you don't have access to this lifecycle.

you could use a <script async></script> instead, https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#attr-async, and perform the fetch with extra headers. It will be executed in parallel with other resources of the page.

Upvotes: 1

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