Node-fetch: TypeError: body used already for - should be able to extract twice

Created on 12 Oct 2018 · 5Comments · Source: node-fetch/node-fetch

I unfortunately am calling a service which may or may not return JSON (or any) body data.

When I do the following, I get the TypeError: body used already for error:

let data = null

try {
  data = await res.json()
} catch (e) {
  data = await res.text()
}

Two things:

I should be able to call .json() / .text() multiple times
If the body is empty, it should just return null instead of throwing on .text()

Source

theogravity

👍28

Most helpful comment

the alternetive is that you only use .text() and then try to parse the json into a object using JSON.parse(text)

Maybe also worth having a look at some of the headers you get back. eg content-length, content-type that match application/json look also if res.ok is true or false before you continue down the path

jimmywarting on 12 Oct 2018

👍24

All 5 comments

you should look into .clone() and be aware of highWaterMark (#386)

(the underlying data is created using stream, which means you can only call json and text once, unless you clone it.)

bitinn on 12 Oct 2018

👍9

the alternetive is that you only use .text() and then try to parse the json into a object using JSON.parse(text)

jimmywarting on 12 Oct 2018

👍24

Thanks, that's a reasonable solution

theogravity on 13 Oct 2018

I wanted to update on this issue - this has bitten us a few times already that we've noted this issue in our code base as to why we do it this way.

There's also a longer discussion about it on:

https://www.reddit.com/r/javascript/comments/a3g9bi/you_may_not_need_axios/eb606rd/

It's not node-fetch's fault here - the fetch spec dictates that a stream is used as the underlying implementation.

theogravity on 3 Jan 2019

👍1

The use-case from the beginning of the post is, imho, very normal and should be supported. I have just tried doing literally the same thing, and I believe many people think the same (probably because we are used to that try { JSON.parse } catch {} paradigm. It is not relevant to consumers of the library that there is some underlying stream etc. as implementation. This should work, all I'm saying...