bug: `toArrayLike()` crashes with zero-valued BigNumber

[gupta-ji6]

Summary

Hi! 👋

Firstly, thanks for your work on this project! 🙂

The toArrayLike() method in react-native-bignumber crashes when called on a BigNumber representing zero. This occurs because the method creates a 0-length buffer and passes it to native code, which causes an immediate native crash.

Environment

• react-native-bignumber: 0.2.3
• Platform: React Native (iOS/Android) 0.74.7
• Impact: Critical - Native app crash

Problem Description

When toArrayLike() is called on a BigNumber with value 0:

1. this.byteLength() returns 0
2. outLen becomes 0
3. A 0-length buffer is created: new ArrayType(0)
4. The 0-length buffer is passed to native.toArrayLike.call()
5. Native code crashes attempting to work with empty buffer

Code Analysis

Current Implementation (Crashing)

Refer -

react-native-bignumber/src/BigNumber.ts

Line 229 in bf2703f

toArrayLike<T extends (len: number) => { buffer: ArrayBuffer }>(

toArrayLike(arrayLike, endian?, len?) {
  const outLen = len != null ? len : this.byteLength();  // ❌ Can be 0
  
  const res = new (arrayLike as any)(outLen);  // ❌ Creates 0-length buffer
  
  native.toArrayLike.call(
    this.internalBigNum,
    res.buffer,  // ❌ 0-length buffer crashes native code
    endian === 'le',
    len || -1
  );
  return res;
}

Standard bn.js Implementation (Working)

Refer - https://github.com/indutny/bn.js/blob/6db7c3818569423b94ebcf2bdff90fcfb9c47f6d/lib/bn.js#L573

BN.prototype.toArrayLike = function toArrayLike (ArrayType, endian, length) {
  var byteLength = this.byteLength();
  var reqLength = length || Math.max(1, byteLength);  // ✅ Ensures minimum 1 byte
  
  var res = allocate(ArrayType, reqLength);
  // ... pure JS implementation (no native crashes)
  return res;
};

Reproduction Steps

Minimal Reproduction

import { BN } from 'react-native-bignumber';

// Create a zero-valued BigNumber
const zeroBN = new BN(0);

// This should crash the app
const buffer = zeroBN.toArrayLike(Buffer);  // 💥 CRASH

Real-world Scenario (Solana Web3.js)

This issue was discovered when using Solana's web3.js library @solana/web3.js@1.95.3 in React Native. It uses bn.js but we have polyfilled it with react-native-bignumber

Refer - https://github.com/solana-foundation/solana-web3.js/blob/16f145d4ba5542488e22c0b71d539c4e36ae65f5/src/publickey.ts#L123

  toBuffer(): Buffer {
    const b = this._bn.toArrayLike(Buffer); // toArrayLike called here
    if (b.length === PUBLIC_KEY_LENGTH) {
      return b;
    }

    const zeroPad = Buffer.alloc(32);
    b.copy(zeroPad, 32 - b.length);
    return zeroPad;
  }

Impact: This makes Solana SOL transfers failures in React Native apps, as the Solana System Program ID 11111111111111111111111111111111 is required for all SOL transfer transactions.

[gupta-ji6]

• Raised a PR with a patch which worked for us while testing solana transactions - fix(toArrayLike): ensure minimum array length in BigNumber for zero length buffers #81