[Bug]: route.fulfill({ json }) replaces a Content-Type set in headers and sets none for null, false, 0 and ''

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Evaluación

Dificultad
2/5
Tiempo estimado
1-3 horas
Aptitud para principiantes
82/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
javascript, typescript
Área
api, testing

Línea de trabajo

El error está en la rama json de _innerFulfill en packages/playwright-core/src/client/network.ts, que establece el Content-Type sin comprobar los encabezados existentes y comprueba la veracidad de options.json. Lee el bloque justo encima, donde el cuerpo se construye con options.json !== undefined, y compara las dos condiciones. El objetivo se cumple cuando json conserva el valor de un Content-Type preestablecido y null, false, 0 y '' envían application/json. Compruébalo con las pruebas de especificación de reproducción tests/fulfill-json.spec.ts en Chromium, Firefox y WebKit.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Version

1.65.1

Steps to reproduce
  1. Clone https://github.com/ondraulehla/playwright-repro-fulfill-json-content-type
  2. npm install
  3. npx playwright install
  4. npx playwright test

The repro is one spec file, tests/fulfill-json.spec.ts. Every request is routed, so it needs no server. Each test does this with a different set of options:

await page.route('http://repro.test/data', route => route.fulfill(options));
const result = await page.evaluate(async () => {
  const response = await fetch('/data');
  return { contentType: response.headers.get('content-type'), body: await response.text() };
});
Expected behavior

The docs for the json option say: "JSON response. This method will set the content type to application/json if not set."

So I'd expect two things:

  1. route.fulfill({ json: { a: 1 }, headers: { 'Content-Type': 'application/problem+json' } }) keeps application/problem+json, because the content type is already set.
  2. route.fulfill({ json: null }) responds with application/json, and so do false, 0 and '', the same way json: 1 does. Playwright already serializes the body for them (null, false, 0, "").
Actual behavior

18 of the 30 tests fail, the same 6 in Chromium, Firefox and WebKit. The 4 control tests pass in all three.

fulfill options Expected Chromium Firefox WebKit
json: { a: 1 }, headers: { 'content-type': 'application/problem+json' } application/problem+json application/json application/json application/json
json: { a: 1 }, headers: { 'Content-Type': 'application/problem+json' } application/problem+json application/json application/json application/json
json: null application/json no header no header text/plain
json: false application/json no header no header text/plain
json: 0 application/json no header no header text/plain
json: '' application/json no header no header text/plain

"no header" means response.headers.get('content-type') is null in the page.

I get the same table on macOS and on Linux (Ubuntu 24.04 in Docker).

Additional context

I found this while looking at how the clients pick the content type after #43205, which was the Python side of the second case (json=[] and json={}).

Both cases come from the same block in _innerFulfill in packages/playwright-core/src/client/network.ts:

if (options.contentType)
  headers['content-type'] = String(options.contentType);
else if (options.json)
  headers['content-type'] = 'application/json';

The json branch doesn't check whether headers already has a content type, and it tests options.json for truthiness. A few lines above, the body is built with options.json !== undefined, which is why the falsy values get a serialized body but no header.

I tried this change against the installed 1.65.1 bundle and all 30 tests pass with it:

else if (options.json !== undefined && !('content-type' in headers))

I'd like to send a PR with the fix and tests if this is approved for a community contribution.

Environment
System:
  OS: macOS 26.6.2
  CPU: (8) arm64 Apple M1 Pro
Binaries:
  Node: 26.5.0 - /opt/homebrew/bin/node
  npm: 11.17.0 - /opt/homebrew/bin/npm
npmPackages:
  @playwright/test: 1.65.1 => 1.65.1
Lenguaje dominante
TypeScript
Estrellas
99.7k
Forks
6.8k
Merge medio
16 h 10 min
PR fusionados (30 d)
301

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