Emulate the keepalive option by doing a synchronous request #700

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1. mislav commented on Apr 17, 2019

   

   mislav

   on Apr 17, 2019

   Last edited by mislav

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   Hi, thank you for the thoughtful feature request.

   There are pros and cons to us implementing this, and I think you've done a good job outlining them. The technical aspect of this isn't hard (it would likely be only 2 lines of code), and it would certainly be useful is keepalive setting could be used in browsers old and new alike.

   However, synchronous XHRs are just bad. There's a reason why browsers are deprecating/disallowing them now. They block the whole thread, freezing up the browser tab until they are done. What if the site that they are making the request to isn't responsive?

   We cannot in good consciousness support people using synchronous XHR.

   My advice: just use sendBeacon when available, with no fallback, since it's supported pretty well across Chrome, Firefox, Safari 11.1+, and Edge 14+.
2. mislav

   closed this as completedon Apr 17, 2019
3. alexturpin commented on Apr 17, 2019

   

   alexturpin

   on Apr 17, 2019

   Last edited by alexturpin

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   Thanks for taking the time to answer!

   sendBeacon has a few shortcomings that make it unusable in our case, and likely for others.

   • It only works for POST requests
   • In Chrome it has a data limit of 64kb

   fetch with keepalive is then the obvious alternative. However, the problem then becomes that I cannot use this polyfill in my application to streamline code because it will silently fail for polyfilled browsers. I find myself having to store whether fetch is natively available or not before including the polyfill, and then using that to determine whether to use fetch with keepalive for native browsers or synchronous XHR for the rest, which definitely isn't ideal. I'm open to suggestions on how to deal with that in a cleaner way.

   At the very minimum, I would suggest adding a notes under the "Caveats" section of the README with other potential solutions. Ideally, the library would throw when keepalive is used with it instead of failing silently.
4. mislav commented on Apr 18, 2019

   

   mislav

   on Apr 18, 2019

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   I find myself having to store whether fetch is natively available or not before including the polyfill, and then using that to determine whether to use fetch with keepalive for native browsers or synchronous XHR for the rest, which definitely isn't ideal.

   Using feature detection is better. It's possible that not all browsers that have initially implemented window.fetch have also implemented keepalive.

   const browserSupportsKeepalive = 'keepalive' in new Request('')
   
   // later:
   if (browserSupportsKeepalive) {
     fetch(url, {keepalive: true})
   } else {
     new XMLHttpRequest
   }

   At the very minimum, I would suggest adding a notes under the "Caveats" section of the README

   We welcome PRs to improve documentation! I would suggest first adding the note about keepalive to the bottom of https://github.github.io/fetch/#caveats (in the gh-pages git branch) and then linking to that section from Caveats in the README.
5. github-actions

   locked as resolved and limited conversation to collaborators on Oct 1, 2020
6. ppichate2531

   added a commit that references this issue on Mar 12, 2021

   Initial commit

   e51904c