node-fetch: reject json() on an empty body by robobun · Pull Request #42689 · oven-sh/bun

robobun · 2026-09-14T08:02:12Z

Problem

With the built-in node-fetch, res.json() on an empty 200 body resolves null. Bun 1.4.2, node-fetch v2 and v3, and Node's fetch reject with a SyntaxError. Code like try { data = await res.json() } catch { data = {} } now gets null. Regression from node-fetch, undici: end the body stream when a body method took the response #42573 (not released).
Cause: the shim's Response inherits the native json(), which resolves null for an empty fetched body (Any::to_json, src/runtime/webcore/Blob.rs:6150, Response.json behavior is different between bun and node #24955). Before node-fetch, undici: end the body stream when a body method took the response #42573 the shim's json() read this.body first. That made the body a stream, and the stream path rejects. node-fetch, undici: end the body stream when a body method took the response #42573 deleted that override.

Fix

json() in src/js/thirdparty/node-fetch.ts is now JSON.parse(await super.text()). That is what node-fetch does, so an empty body rejects with the JSON.parse("") error, the same message as in 1.4.2.
The result no longer depends on whether res.body was read before. No stream is created, unlike with the old override.
Reject json() on an empty body with the JSON.parse SyntaxError #33658 removes the null in the native code, also for global fetch() and Bun.serve() requests. That wider change still needs a decision. This override is correct either way.
Verified: test/js/node/http/node-fetch.test.js (4 new tests, 3 fail with main's src/). Other suites are in the Notes.

Background

node-fetch in Bun is a built-in shim. Its Response extends the native Response. res.body is a node Readable around the web stream.
A fetched body stays a byte buffer until something reads .body. The native json() returns null for a zero-length buffer. A stream is read to the end and parsed, and zero bytes reject.

Notes

Reproduction.

import http from "node:http";
import nodeFetch from "node-fetch";
const srv = http.createServer((q, s) => {
  if (q.url === "/chunked0") { s.setHeader("transfer-encoding", "chunked"); return s.end(); }
  s.setHeader("content-length", "0"); s.end();
});
await new Promise(r => srv.listen(0, "127.0.0.1", r));
const base = "http://127.0.0.1:" + srv.address().port;
const show = p => p.then(v => "RESOLVED " + JSON.stringify(v), e => "rejected " + e.name + ": " + e.message);
for (const path of ["/empty", "/chunked0"]) {
  console.log(path, await show(nodeFetch(base + path).then(r => r.json())));
  console.log(path, "body first", await show(nodeFetch(base + path).then(r => { void r.body; return r.json(); })));
}
srv.close();

Results, json() on an empty 200 (Content-Length: 0 and an empty chunked body give the same result):

build	`json()`	`void res.body`, then `json()`
1.4.2	rejects `JSON Parse error: Unexpected EOF`	same
main before #42573 (`09bb546`)	rejects `Unexpected end of JSON input`	same
main (`5fce36e`)	resolves `null`	rejects `Unexpected end of JSON input`
this PR	rejects `JSON Parse error: Unexpected EOF`	same
`node-fetch` 3.3.2 on Node 26	rejects `SyntaxError: Unexpected end of JSON input`
`node-fetch` 2.7.0 on Node 26	rejects `FetchError` (`invalid-json`)

Only json() differs. On main I ran json, text, arrayBuffer, blob, formData, buffer, bytes over ten bodies (empty, JSON, text, BOM only, BOM plus JSON, form data, invalid JSON, non-ASCII), each with and without void res.body first. The three empty-body json() cells are the only ones that differ. With this PR none differ. The other four overrides that #42573 deleted stay deleted.

Messages. For invalid JSON that is not empty, JSON.parse and the native json() give the same JSC message (JSON Parse error: Expected '}'). A 204 and a Response made with "" rejected with the hardcoded Unexpected end of JSON input before. They now reject with the JSON.parse("") message as well.

Speed. await res.json() against JSON.parse(await res.text()) with the global fetch on a release build, loopback: 38 µs both ways for a 60 byte body, 18 ms against 16 ms for a 1.9 MB body.

Not in this PR. undici.fetch is Bun.fetch, so it resolves null on every build (#24955, #33658). undici.request().body.json() rejects, because the wrapper reads response.body in its constructor.

Suites run on the debug build: node-fetch.test.js, node-fetch-cjs.test.js, node-fetch-primordials.test.ts, undici.test.ts, undici-primordials.test.ts, node-http-agent-free-socket.test.ts, regression 26225, 014865, 04947.

[auto-merge] gate passed · iteration 0 · 2 files touched

fails on main (without fix)

ASAN without fix: 3 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/node/http/node-fetch.test.js
bun test v1.4.3 (09bb54630)

test/js/node/http/node-fetch.test.js:
(pass) node-fetch [13.19ms]
(pass) node-fetch Headers.raw() [6.44ms]
(pass) node-fetch.fetch fetches [52.15ms]
(pass) node-fetch.default fetches [11.78ms]
(pass) node-fetch.default.default fetches [12.91ms]
(pass) isomorphic-fetch.fetch fetches [12.47ms]
(pass) isomorphic-fetch.default.fetch fetches [10.39ms]
(pass) isomorphic-fetch.default fetches [10.63ms]
(pass) @vercel/fetch.default fetches [20.31ms]
(pass) node-fetch uses node streams instead of web streams [236.92ms]
(pass) node-fetch gives a fetched response without a body an empty stream [69.29ms]
(pass) node-fetch body stream ends without an error after arrayBuffer() consumed the response [83.38ms]
(pass) node-fetch body stream ends without an error after blob() consumed the response [40.62ms]
(pass) node-fetch body stream ends without an error after buffer() consumed the response [45.89ms]
(pass) node-fetch body stream ends without an error after bytes() consumed the respons
... (truncated)

release without fix: 17 FAILED
bun test v1.4.3-canary.1 (09bb54630)

test/js/node/http/node-fetch.test.js:
(pass) node-fetch [0.07ms]
(pass) node-fetch Headers.raw() [0.14ms]
(pass) node-fetch.fetch fetches [2.88ms]
(pass) node-fetch.default fetches [0.76ms]
(pass) node-fetch.default.default fetches [0.73ms]
(pass) isomorphic-fetch.fetch fetches [1.16ms]
(pass) isomorphic-fetch.default.fetch fetches [0.62ms]
(pass) isomorphic-fetch.default fetches [0.95ms]
(pass) @vercel/fetch.default fetches [1.32ms]
(pass) node-fetch uses node streams instead of web streams [4.73ms]
(pass) node-fetch gives a fetched response without a body an empty stream [2.13ms]
185 | 
186 |     for (const [action, expected] of bodyActions) {
187 |       const res = await fetch2(url);
188 |       const body = res.body;
189 |       await res[method]();
190 |       expect(await eventsUntilClose(body, () => body[action]())).toEqual(expected);
                                                                       ^
error: expect(received).toEqual(expected)

  [
-   "end",
+   [TypeError: Invalid state: ReadableStream is locked],
    "close",
  ]

- Expected  - 1
+ Received  + 1

      at <anonymous> (/workspace/bun/test/js/node/h
... (truncated)

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/node/http/node-fetch.test.js
bun test v1.4.3 (09bb54630)

test/js/node/http/node-fetch.test.js:
(pass) node-fetch [14.45ms]
(pass) node-fetch Headers.raw() [5.93ms]
(pass) node-fetch.fetch fetches [54.62ms]
(pass) node-fetch.default fetches [12.16ms]
(pass) node-fetch.default.default fetches [10.82ms]
(pass) isomorphic-fetch.fetch fetches [12.44ms]
(pass) isomorphic-fetch.default.fetch fetches [9.75ms]
(pass) isomorphic-fetch.default fetches [10.72ms]
(pass) @vercel/fetch.default fetches [20.62ms]
(pass) node-fetch uses node streams instead of web streams [267.27ms]
(pass) node-fetch gives a fetched response without a body an empty stream [78.97ms]
(pass) node-fetch body stream ends without an error after arrayBuffer() consumed the response [82.35ms]
(pass) node-fetch body stream ends without an error after blob() consumed the response [40.83ms]
(pass) node-fetch body stream ends without an error after buffer() consumed the response [48.47ms]
(pass) node-fetch body stream ends without an error after bytes() consumed the response
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped) in 614ms (unchanged)
ninja: Entering directory `/workspace/bun/build/release'
[1/21] gen JS modules (bundle-modules)
Preprocess modules (6751ms)
Bundle modules (40ms)
Postprocesss modules (20ms)
Bundle Functions (480ms)
Generate Code (37ms)

[7.34s] Bundled "src/js" for production
  2600 kb
  197 internal modules
  13 native modules
  50 internal functions across 16 files
[1/5] cargo bun_runtime → libbun_runtime.a
�[1m�[92m   Compiling�[0m bun_runtime v0.0.0 (/workspace/bun/src/runtime)
�[1m�[92m    Finished�[0m `release` profile [optimized + debuginfo] target(s) in 4m 44s
[2/5] link bun-profile
[4/5] strip bun
[4/5] bun-profile --revision
1.4.3-canary.1+554249401
[build] done
bun test v1.4.3-canary.1 (554249401)

test/js/node/http/node-fetch.test.js:
(pass) node-fetch [0.07ms]
(pass) node-fetch Headers.raw() [0.10ms]
(pass) node-fetch.fetch fetches [2.73ms]
(pass) node-fetch.default fetches [0.61ms]
(pass) node-fetch.default.default fetches [0.56ms]
(pass) isomorphic-fetch.fetch fetches [0.49ms]
(pass) isomorphic-fetch.default.fetch fetches [0.66ms]
(pass) isomorphic-fetch.defaul
... (truncated)

diff hotspot

src/js/thirdparty/node-fetch.ts      |  8 ++++++++
 test/js/node/http/node-fetch.test.js | 38 ++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                                  reads  edits  tests
src/js/thirdparty/node-fetch.ts           1      3      9
test/js/node/http/node-fetch.test.js      1      2      9

The node-fetch Response inherits the native json(), which resolves null for an empty fetched body. node-fetch's json() is JSON.parse(await this.text()), so it rejects with a SyntaxError. Before #42573 the json() override read `this.body` first. That sent the call down the stream path, which rejects. #42573 deleted the override. json() is now JSON.parse of the native text(), as in node-fetch.

robobun · 2026-09-14T08:02:45Z

Status

Reproduced on a debug build of main (5fce36e). nodeFetch(url).then(r => r.json()) against a node:http server that answers 200 with Content-Length: 0, or with an empty chunked body, resolves null. The same script rejects with a SyntaxError on 1.4.2 and on the canary from before node-fetch, undici: end the body stream when a body method took the response #42573 (09bb546).
With this branch the call rejects, with and without a read of res.body first, and on a clone.
Run the test: bun bd test test/js/node/http/node-fetch.test.js. With main's src/, 3 of the 4 new tests fail.

coderabbitai · 2026-09-14T08:04:51Z

Walkthrough

The change overrides Response.json() in node-fetch to parse response text directly. Tests cover empty HTTP responses across body-access patterns and confirm that JSON null remains valid.

Changes

Response JSON parsing

Layer / File(s)	Summary
Response JSON override `src/js/thirdparty/node-fetch.ts`	Adds a cached `JSON.parse` reference and overrides `Response.json()` to parse `await super.text()`. Empty bodies reject.
HTTP response test coverage `test/js/node/http/node-fetch.test.js`	Tests empty responses for zero-length, chunked, 204, pre-read, and cloned responses. Tests that JSON `null` resolves to `null`.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 55424

The response parsing change has targeted coverage for empty and null bodies. The remaining test-structure convention should be corrected, but it does not create a merge-blocking runtime risk.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary change: rejecting JSON parsing for empty node-fetch response bodies.
Description check	✅ Passed	The description clearly explains the problem, root cause, fix, expected behavior, and verification results. It does not use the template headings exactly, but it provides the required information in e…

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Actionable comments posted: 1

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Inline comments:
In `@test/js/node/http/node-fetch.test.js`:
- Around line 283-287: Update the parameterized test around “node-fetch json()
rejects on %s like JSON.parse('')” to use describe.each() for the matrix, then
place the existing async assertion inside a nested test() while preserving all
cases and expectations.

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📒 Files selected for processing (2)

src/js/thirdparty/node-fetch.ts
test/js/node/http/node-fetch.test.js

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claude

LGTM — focused regression fix that mirrors upstream node-fetch's json() exactly.

What was reviewed:

The json() override calls super.text() (native, not user-overridable) and JSON.parse is captured at module load, matching the tamper-resistance convention used elsewhere in this file.
Comment cites the upstream node-fetch source line and the tracking issue for the native-json() divergence.
Tests cover the variant matrix (three empty-body shapes × direct/body-first/clone), derive the expected error from JSON.parse("") at runtime rather than hardcoding a message, and include a positive case for a literal null body; await using http.createServer and once(..., "listening") follow existing patterns in the suite.

Extended reasoning...

Overview

This PR restores node-fetch-compatible Response#json() semantics in Bun's built-in node-fetch shim. A prior change (#42573) removed a json() override on the shim's Response subclass, causing it to inherit the native Web Response#json(), which resolves null for an empty fetched body instead of rejecting. The fix adds a 3-line override — async json() { return JSONParse(await super.text()); } — that is byte-for-byte what upstream node-fetch v3 does, and captures JSON.parse into a module-level const at load time. Tests are added to the existing test/js/node/http/node-fetch.test.js.

Security risks

None. This is a thirdparty-compat shim change to how an already-received response body is parsed. No auth, crypto, filesystem, or network-origination paths are touched. JSON.parse is captured at module load so userland tampering with the global cannot redirect the parse; super.text() goes straight to the native WebResponse prototype method rather than through anything user-overridable on the instance.

Level of scrutiny

Low-to-moderate. The source change is a handful of lines in a compat shim, restoring pre-regression behavior by copying the reference implementation's exact approach (with the upstream line cited in the comment, per the Node/Web compat convention). It follows the file's existing patterns: module-load capture of globals (alongside nativeFetch, WebResponse, etc.), a short method on the Response subclass next to the analogous buffer() override, and a comment linking the upstream source and the tracking issue.

Other factors

The tests are well-constructed against REVIEW.md's checklist: test.each over the three empty-body shapes (Content-Length: 0, empty chunked via node:http, 204), a combined-object .toEqual covering .json(), body-touched-first, and .clone().json() in one assertion, port: 0 with local servers only, await using on the http.Server (an established pattern in test/js/node/http/), once(server.listen(...), "listening") to await readiness rather than sleeping, and the expected rejection derived at runtime from JSON.parse("") so the test isn't brittle to JSC message wording. A positive test confirms a body containing the JSON text null still resolves null, guarding against over-rejection. No CODEOWNERS cover the changed paths, the bug hunt exited on dry_streak with no findings, and there are no outstanding reviewer objections in the timeline.

robobun · 2026-09-14T08:35:47Z

^{Updated 1:35 AM PT - Sep 14th, 2026}

✅ @robobun, your commit 554249401f96511740c21f082bbe505c43a0b117 passed in Build #115442! 🎉

🧪 To try this PR locally:

bunx bun-pr 42689

That installs a local version of the PR into your bun-42689 executable, so you can run:

bun-42689 --bun

robobun assigned Jarred-Sumner Sep 14, 2026

github-actions Bot added the claude label Sep 14, 2026

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Comment thread test/js/node/http/node-fetch.test.js

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Jarred-Sumner merged commit 731a117 into main Sep 14, 2026
7 of 8 checks passed

Jarred-Sumner deleted the robobun/6dbb3ca1/node-fetch-json-empty-body branch September 14, 2026 19:07

usrbinkat pushed a commit to usrbinkat/bun that referenced this pull request Sep 15, 2026

node-fetch: reject json() on an empty body (oven-sh#42689)

b5c8df9

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node-fetch: reject json() on an empty body - #42689

node-fetch: reject json() on an empty body#42689
Jarred-Sumner merged 1 commit into
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robobun/6dbb3ca1/node-fetch-json-empty-body

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