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Fix TypeError in CheckStringCoercion typeName() for null-prototype objects in DEV - #37900

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prasad2512:fix-check-string-coercion-type-name-37881
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Fix TypeError in CheckStringCoercion typeName() for null-prototype objects in DEV#37900
prasad2512 wants to merge 1 commit into
react:mainfrom
prasad2512:fix-check-string-coercion-type-name-37881

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@prasad2512

@prasad2512 prasad2512 commented Oct 19, 2026

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Summary

Fixes #37881

In development mode (__DEV__), typeName(value) in packages/shared/CheckStringCoercion.js constructs a human-readable diagnostic type string when a prop, key, attribute, or style value throws during string coercion ('' + value).

Currently, typeName(value) accesses (value as any).constructor.name:

const type =
  (hasToStringTag && (value as any)[Symbol.toStringTag]) ||
  (value as any).constructor.name ||
  'Object';

When value is created without a prototype (e.g. Object.create(null) or { __proto__: null }), (value as any).constructor is undefined. Accessing .name on undefined causes an unhandled runtime error:

TypeError: Cannot read properties of undefined (reading 'name')
    at typeName (packages/shared/CheckStringCoercion.js)

This crashes React's internal DEV helper before React can log the diagnostic error message and troubleshooting recommendations.

This PR safely checks for constructor existence before accessing .name:

const constructor = (value as any).constructor;
const type =
  (hasToStringTag && (value as any)[Symbol.toStringTag]) ||
  (constructor && constructor.name) ||
  'Object';

When constructor is undefined or null, it gracefully falls back to 'Object'.

How did you test this change?

  • Added a regression test in packages/react-dom/src/__tests__/ReactIdentity-test.js verifying that passing a null-prototype object with a throwing valueOf does not crash typeName(), properly prints the diagnostic error message (The provided key is an unsupported type Object. This value must be coerced to a string before using it here.), and throws the expected coercion exception.

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meta-cla Bot commented Oct 19, 2026

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github-actions Bot commented Oct 19, 2026 •
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A size report will appear here when the build finishes.

Generated by sizebot against c55c1b5

@prasad2512
prasad2512 force-pushed the fix-check-string-coercion-type-name-37881 branch from d81879c to 4c7b4a2 Compare October 19, 2026 17:52
@meta-cla meta-cla Bot added the CLA Signed label Oct 21, 2026
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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

@prasad2512
prasad2512 force-pushed the fix-check-string-coercion-type-name-37881 branch from 4c7b4a2 to c55c1b5 Compare October 28, 2026 02:09
@prasad2512

prasad2512 commented Oct 28, 2026

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Hi @eps1lon, rebased onto latest main with CLA verified. Includes regression test in ReactIdentity-test for null-prototype objects in typeName(). Ready for review when convenient. Thanks!

@prasad2512
prasad2512 requested a review from willbyers1 October 28, 2026 02:11

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Bug: TypeError in CheckStringCoercion typeName() for null-prototype objects in DEV

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