fix: reject fractional wei in toWei

CHANGELOG.md

@@ -14,6 +14,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  - `convertMnemonicToWordlistIndices` converts a mnemonic string into little-endian `Uint16` wordlist indices encoded as a `Uint8Array`.
 - Add `@metamask/scure-bip39` as a dependency ([#300](https://github.com/MetaMask/utils/pull/300))
 
+### Fixed
+
+- `toWei` now rejects fractional wei values (e.g. `toWei('0.5', 'wei')`), which previously returned an incorrect whole-wei value instead of throwing ([#303](https://github.com/MetaMask/utils/pull/303))
+ - This is a caller-visible change: `toWei('0.0', 'wei')` now throws instead of returning `0n`, consistent with how the library already rejects an over-precision fraction for every other unit.
+
 ## [11.11.0]
 
 ### Added

src/unitsConversion.test.ts

@@ -151,7 +151,10 @@ describe('getValueOfUnit', () => {
 describe('toWei', () => {
  it('should handle edge cases', () => {
  expect(toWei(0, 'wei').toString(10)).toBe('0');
- expect(toWei('0.0', 'wei').toString(10)).toBe('0');
+ // wei has zero decimal places, so an explicit fractional digit (even a
+ // literal zero) is "too many decimal places", same as it would be for
+ // any other unit given more fractional digits than it supports.
+ expect(() => toWei('0.0', 'wei')).toThrow(Error);
  expect(toWei('.3', 'ether').toString(10)).toBe('300000000000000000');
  expect(() => toWei('.', 'wei')).toThrow(Error);
  expect(() => toWei('1.243842387924387924897423897423', 'ether')).toThrow(
@@ -160,6 +163,21 @@ describe('toWei', () => {
  expect(() => toWei('8723.98234.98234', 'ether')).toThrow(Error);
  });
 
+ it('should reject fractional wei (wei has zero decimal places)', () => {
+ // Regression test: unitLengths used to compute `value.length - 1 || 1`,
+ // so wei's baseLength was wrongly coerced from 0 to 1, letting a single
+ // fractional digit silently pass through as whole wei.
+ expect(() => toWei('0.5', 'wei')).toThrow('too many decimal places');
+ expect(() => toWei('0.9', 'wei')).toThrow('too many decimal places');
+
+ // Whole wei values are unaffected by the fix.
+ expect(toWei('5', 'wei')).toBe(BigInt(5));
+ expect(toWei('0', 'wei')).toBe(BigInt(0));
+
+ // Sanity-check another unit (baseLength 9) still works after the fix.
+ expect(toWei('1.5', 'gwei')).toBe(BigInt('1500000000'));
+ });
+
  it('should handle BigInt inputs with fast path optimizations', () => {
  // Fast path: BigInt + 'wei' unit (should return input directly)
  expect(toWei(BigInt(123), 'wei')).toBe(BigInt(123));

src/unitsConversion.ts

@@ -68,7 +68,7 @@ const unitMapBigInt = Object.fromEntries(
 ) as Record<EthereumUnit, bigint>;
 
 const unitLengths = Object.fromEntries(
- Object.entries(unitMap).map(([key, value]) => [key, value.length - 1 || 1]),
+ Object.entries(unitMap).map(([key, value]) => [key, value.length - 1]),
 ) as Record<EthereumUnit, number>;
 
 const NUMBER_REGEX = /^-?[0-9.]+$/u;
@@ -266,16 +266,17 @@ export function toWei(
  if (!whole) {
  whole = '0';
  }
- if (!fraction) {
+
+ if (fraction) {
+ if (fraction.length > baseLength) {
+ throw new Error(
+ `While converting number ${etherInput} to wei, too many decimal places`,
+ );
+ }
+ fraction = fraction.padEnd(baseLength, '0');
+ } else {
  fraction = '0';
  }
- if (fraction.length > baseLength) {
- throw new Error(
- `While converting number ${etherInput} to wei, too many decimal places`,
- );
- }
-
- fraction = fraction.padEnd(baseLength, '0');
 
  const wholeBigInt = BigInt(whole);
  const fractionBigInt = BigInt(fraction);