HEAD response with a JSON content-type throws "Unexpected end of JSON input"

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
82/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, node.js
Domain
backend-api-design

Research direction

The issue points to _handleResponse in lib/httpClient.js (lines 105–114). Read how it chooses to parse response bodies, then check the existing tests for httpClient.head(url) and httpClient(url, {method: 'HEAD'}). Done when both forms resolve successfully with response.data undefined for a JSON content type.

Written by the indexing model from the issue text.

Description

A HEAD request to an endpoint that serves JSON rejects with SyntaxError: Unexpected end of JSON input, even though the request succeeded.

A HEAD response carries the same headers as the GET, including Content-Type: application/json, but has no body. _handleResponse decides whether to parse from the content-type alone, so it calls response.json() on the empty body:

https://github.com/digitalbazaar/http-client/blob/main/lib/httpClient.js#L105-L114

Reproduction

Seen with 4.4.0 on Node 24.

import {createServer} from 'node:http';
import {httpClient} from '@digitalbazaar/http-client';

const server = createServer((req, res) => {
  const body = JSON.stringify({ok: true});
  res.writeHead(200, {
    'content-type': 'application/json',
    'content-length': Buffer.byteLength(body)
  });
  // a HEAD response carries the GET's headers and no body
  res.end(req.method === 'HEAD' ? undefined : body);
});
await new Promise(resolve => server.listen(0, resolve));
const url = `http://localhost:${server.address().port}/`;

console.log('GET ', (await httpClient.get(url)).data);
try {
  console.log('HEAD', (await httpClient.head(url)).status);
} catch(e) {
  console.log('HEAD threw:', e.name, '-', e.message);
}
server.close();

Output:

GET  { ok: true }
HEAD threw: SyntaxError - Unexpected end of JSON input
Expected

The HEAD resolves with the response, and response.data is undefined.

Workaround

Passing parseBody: false avoids it. That only helps a caller that controls the options, which is not the case when the request is made through a wrapper such as ezcap's ZcapClient.request().

Suggested fix

Skip the body parse when the request method is HEAD, both for httpClient.head(url) and for the direct httpClient(url, {method: 'HEAD'}) form. Happy to send a PR.

Dominant language
JavaScript
Stars
2
Forks
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