fix: Get responseBodySize from body if content-length is missing

[metal-messiah]

Addresses an issue where AjaxRequest events inaccurately reported a responseSize of 0 or undefined due to a lack of fallback mechanisms and overly restrictive falsy checks. Introduced a fallback mechanism that calculates response sizes from captured payloads for Fetch and XHR requests when the content-length header is missing.

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Overview

This pull request resolves multiple edge cases where Ajax request responseSize tracking failed or defaulted to undefined instead of accurately reflecting the payload size.

Key Changes:

• Fixed dataSize() Utility Bug: Adjusted src/common/util/data-size.js to ensure empty strings return 0 instead of falling through to undefined.
• Fetch & XHR Fallbacks: Added fallback logic to onFetchDone and XHR end handlers to measure the captured response body payload when the content-length header or primary detection is missing (operational when payload capture is enabled).
• Corrected Falsy Value Checks: Replaced restrictive if (!value) blocks with explicit if (value !== undefined) validation to ensure valid 0 sizes (e.g., 204 No Content responses) are not blocked or omitted.
• Modernized Component Testing: Replaced old mock-based tests in instrument.test.js with integration tests utilizing real XMLHttpRequest objects to execute actual instrumentation flows.

Related Issue(s)

https://new-relic.atlassian.net/browse/NR-446528

Testing

Execute the following commands in your local development environment to verify the fixes and ensure no regressions exist:

# Run dataSize unit tests
npm test -- tests/unit/common/util/data-size.test.js

# Run ajax instrumentation component tests
npm test -- tests/components/ajax/instrument.test.js

# Run ajax e2e tests
npm test -- tests/specs/ajax/

[github-actions]

Supportability Metric Usage Changes:

No matching changes found

Supportability Metrics .md File Changes:

supportability_metrics.md was changed? false

New supportability metrics require changes to supportability_metrics.md, as well as a new PR to Angler. Please ensure an Angler PR is created and linked to this PR.

[github-actions]

Asset Size Report

Merging this pull request will result in the following asset size changes:

Agent	Asset	Previous Size	New Size	Diff
lite	loader	26.79 kB / 10.24 kB (gzip)	26.79 kB / 10.24 kB (gzip)	0% / 0% (gzip)
lite	async-chunk	58.92 kB / 19.76 kB (gzip)	58.92 kB / 19.76 kB (gzip)	0% / 0% (gzip)
pro	loader	66.48 kB / 23.42 kB (gzip)	66.61 kB / 23.46 kB (gzip)	0.2% / 0.18% (gzip)
pro	async-chunk	96.37 kB / 30.65 kB (gzip)	96.37 kB / 30.65 kB (gzip)	0% / 0% (gzip)
spa	loader	68.87 kB / 24.19 kB (gzip)	69.01 kB / 24.24 kB (gzip)	0.19% / 0.19% (gzip)
spa	async-chunk	108.52 kB / 33.76 kB (gzip)	108.52 kB / 33.76 kB (gzip)	0% / 0% (gzip)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.45%. Comparing base (9121c6b) to head (6c09817).

Files with missing lines	Patch %	Lines
src/features/ajax/instrument/index.js	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1793      +/-   ##
==========================================
+ Coverage   88.90%   89.45%   +0.55%     
==========================================
  Files         213      213              
  Lines        7859     7864       +5     
  Branches     1654     1657       +3     
==========================================
+ Hits         6987     7035      +48     
+ Misses        776      746      -30     
+ Partials       96       83      -13     

Flag	Coverage Δ
unit-tests	84.78% <66.66%> (+1.12%)	⬆️

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[ellisong]

looks fine to me if tests pass

[github-actions]

Last ran on June 15, 2026 20:05:43 CDT
Checking merge of (6c09817) into main (9121c6b)