fix(network): request.postData() returns null for empty string body override

[tsushanth]

Summary

request.postData() incorrectly returns null when the POST body is set to an empty string via route.continue({ postData: '' }) or route.fallback({ postData: '' }).

Root cause

In packages/playwright-core/src/client/network.ts, the postData() method uses || null at the end:

// before
postData(): string | null {
  return (this._fallbackOverrides.postDataBuffer || this._initializer.postData)?.toString('utf-8') || null;
}

When the override is set to an empty string, Buffer.from('', 'utf-8') is stored in _fallbackOverrides.postDataBuffer. Since an empty Buffer is a truthy object, the left-hand side resolves correctly to the empty buffer, and .toString('utf-8') yields ''. However, '' || null evaluates to null, discarding the empty-string body.

This is inconsistent with postDataBuffer(), which returns the correct empty Buffer for the same input.

Fix

Switch the trailing || null to ?? null (nullish coalescing), which only falls back to null when the value is null or undefined, not when it is an empty string:

// after
postData(): string | null {
  return (this._fallbackOverrides.postDataBuffer ?? this._initializer.postData)?.toString('utf-8') ?? null;
}

The inner || is similarly changed to ?? so that a zero-length override buffer isn't accidentally skipped in favour of the original request's body.

Reproduction

test('postData returns empty string for empty body override', async ({ page }) => {
  await page.route('**/api', async route => {
    await route.continue({ postData: '' });
  });

  page.on('request', request => {
    // Before fix: null  After fix: ''
    expect(request.postData()).toBe('');
  });

  await page.evaluate(() => fetch('/api', { method: 'POST', body: 'original' }));
});

[dcrousso]

this looks like a good fix (minus the \n issue) but it needs a test

[dcrousso]

oops? i think you may have accidentally replaced all \n with an actual newline elsewhere in this file

[tsushanth]

Thanks for catching those — addressed in the latest push:

• Restored the three escaped \n sequences that were accidentally converted to literal newlines (.join('\n'), rewriteErrorMessage, get() header join)
• Added a test in tests/page/page-request-continue.spec.ts that overrides a POST body with '' and asserts request.postData() returns '' rather than null

[dcrousso]

i dont think this will work as im pretty sure it runs before the route applies

[github-actions]

Test results for "MCP"

3 failed
❌ [chrome] › mcp/annotate.spec.ts:57 › should capture multiple screenshots in one annotation @mcp-windows-latest-chrome
❌ [chrome] › mcp/annotate.spec.ts:110 › should abort annotation when last screenshot is removed @mcp-windows-latest-chrome
❌ [webkit] › mcp/cli-killall.spec.ts:42 › kill-all kills filtered dashboard pid @mcp-ubuntu-latest-webkit

7757 passed, 1249 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

9 failed
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @frozen-time-library-chromium-linux
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @chromium-ubuntu-22.04-arm-node20
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @realtime-time-library-chromium-linux
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @chromium-ubuntu-22.04-node24
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @chromium-ubuntu-22.04-node20
❌ [chromium-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @chromium-ubuntu-22.04-node22
❌ [firefox-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @firefox-ubuntu-22.04-node20
❌ [webkit-page] › page/page-request-continue.spec.ts:1014 › postData should return empty string when overriding body with empty string @webkit-ubuntu-22.04-node20
❌ [playwright-test] › worker-index.spec.ts:324 › should respect project.workers>1 @windows-latest-node22

4 flaky

⚠️ [chromium-library] › library/video.spec.ts:664 › screencast › should capture full viewport `@frozen-time-library-chromium-linux`
⚠️ [chromium-library] › library/browsercontext-page-event.spec.ts:173 › should work with Ctrl-clicking `@chromium-ubuntu-22.04-arm-node20`
⚠️ [chromium-library] › library/video.spec.ts:736 › screencast › should work with video+trace `@chromium-ubuntu-22.04-node22`
⚠️ [firefox-library] › library/inspector/cli-codegen-3.spec.ts:255 › cli codegen › should generate frame locators (4) `@firefox-ubuntu-22.04-node20`

49987 passed, 1189 skipped

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Merge workflow run.