Intl.NumberFormat.format() throws "Cannot convert BigInt to number" when passed a bigint on Android

[brien-crean]

Bug Description

Intl.NumberFormat.prototype.format() throws TypeError: Cannot convert BigInt to number when passed a bigint argument on Android. The same code works correctly in V8 (Chrome) and SpiderMonkey (Firefox).

• I have run gradle clean and confirmed this bug does not occur with JSC
• The issue is reproducible with the latest version of React Native.

Hermes git revision (if applicable): bundled with React Native 0.81.5
React Native version: 0.81.5
OS: Android
Platform: arm64-v8a

Steps To Reproduce

1. Create a React Native app with Hermes enabled (default for RN 0.81.5)
2. Run on an Android device or emulator
3. Execute any of the following:

// All three crash with: TypeError: Cannot convert BigInt to number
new Intl.NumberFormat().format(BigInt('2'))
new Intl.NumberFormat('en-US').format(BigInt('2'))
new Intl.NumberFormat().format(2n)

The crash occurs regardless of whether the bigint is created via the BigInt() constructor or a bigint literal (2n), and regardless of whether a locale argument is provided.

Workaround:

new Intl.NumberFormat().format(Number(BigInt('2'))) // ✅ returns "2"

The Expected Behavior

All three expressions should return "2" without throwing, as bigint is a valid argument per ECMA-402 and works correctly on V8 and SpiderMonkey.

Related open issues:

• Intl.NumberFormat can't always display the number's sign #1317 — Intl.NumberFormat signDisplay can't always display sign
• Intl.NumberFormat on Android with style: 'currency' and signDisplay: 'always' truncates currency sign for positive values #789 — Intl.NumberFormat signDisplay: 'always' truncates currency sign on Android

[tmikov]

Unfortunately this seems to require end to end support of arbitrary precision integers in Intl. Our Intl implementation was added before BigInt, so it is not supported.

To make things worse, nobody on the team is an expert on Intl, and as we have stated publicly, we can't meaningfully maintain it.

With that said, I have assigned the task to Copilot. Let's see what it comes up with.

[tmikov]

Copilot failed miserably. Claude Code was able to get something directionally better in #1930, but we will see...

[brien-crean]

@tmikov Thanks very much for looking into this! Very much appreciated. For now we're wrapping in Number before passing to format() as a workaround.