Better error when trying to .json a Resource that's not JSON.

[matthew-andrews]

It would be nice if there were a better error when trying to .json a Resource that's not JSON.

Currently in two implementations (this polyfill and Chrome Service Workers) trying to fetch and .json() a non JSON resource throws a not particularly helpful error:-

fetch("//some.thing/not-a-json-resource")
  .then(function(res) {
    return res.json();
  })
  .catch(function(err) {
    console.log(err); // => Unexpected token c
  });

It would make for more less unpleasant debugging / error aggregation if it threw an error along the lines of “Failed to parse {{url}} as JSON”… or something like that…

I know errors aren't spec'd as such so it's not something that should/could be fixed in the spec — but I will raise as an issue in Chrome as well so that GitHub & Chrome's fetches can stay aligned but I think it's something that should be improved and I'm sure you have an idea of what it might look like…

[matthew-andrews]

Chrome Bug here: https://code.google.com/p/chromium/issues/detail?id=445041

[josh]

Yeah, that's kinda lame.

Thanks for opening the issue on the Chrome side. I was just recently trying
to align us to any Chrome differences.
On Wed, Dec 24, 2014 at 6:46 AM Matt Andrews notifications@github.com
wrote:

Chrome Bug here:
https://code.google.com/p/chromium/issues/detail?id=445041&thanks=445041&ts=1419425155

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#47 (comment).

[josh]

Have any specific ideas for how we could improve this? Or should we just wait it out until Chrome makes the first move?

[dgraham]

I think this error is pretty good. "SyntaxError" and "unexpected token" are good clues that's it's invalid JSON. Then you can inspect the response and see it's HTML or other non-JSON data.

[matthew-andrews]

for what it's worth i'm playing with ideas here:-
https://github.com/matthew-andrews/fetchres#usage-node