Convert swap amounts without floating-point errors

Keep a swap amount as text until you convert it directly to an integer. Multiplying a JavaScript Number by a power of ten can lose precision before BigInt ever receives the result.

A conversion contract

Define accepted input syntax first. A form may permit an optional decimal point and decimal digits while rejecting negatives, exponent notation and thousands separators. Localized display is a separate concern: normalize a recognized locale before parsing rather than guessing whether a comma is a decimal separator.

For a hypothetical token with six decimals, split 3.125 into whole 3 and fractional 125, pad the fraction to six places and combine them into 3125000 base units. If the fractional part contains more than six significant decimal places, show a validation error. The ERC-20 decimals convention defines this display relationship; it does not require every token to use eighteen decimals.

Example cases for a six-decimal asset

"3.125"      → "3125000"

"0.000001"   → "1"

"0.0000001"  → reject excess precision

"1e6"        → reject unsupported syntax

Preserve exact values after parsing

Use integer arithmetic for balances, allowance comparisons and minimum amounts. Format an integer back into a decimal string for display. BigInt has separate arithmetic and serialization rules; convert amounts to decimal strings at a JSON boundary instead of assuming JSON.stringify accepts native BigInt values.

Also validate the provider's expected range. A positive integer that exceeds a field's width is still invalid. Your fixtures should include a very large amount, the smallest supported unit, leading zeros and an empty field. Compare round trips by exact integer equality, not by a rounded screen label.