metamask-extension/ui/app/pages/send/send-content/send-amount-row/amount-max-button/amount-max-button.utils.js

import {
  multiplyCurrencies,
  subtractCurrencies,
} from '../../../../../helpers/utils/conversion-util'
import { addHexPrefix } from '../../../../../../../app/scripts/lib/util'

export function calcMaxAmount({ balance, gasTotal, sendToken, tokenBalance }) {
  const { decimals } = sendToken || {}
  const multiplier = Math.pow(10, Number(decimals || 0))

  return sendToken
    ? multiplyCurrencies(tokenBalance, multiplier, {
        toNumericBase: 'hex',
        multiplicandBase: 16,
      })
    : subtractCurrencies(addHexPrefix(balance), addHexPrefix(gasTotal), {
        toNumericBase: 'hex',
      })
}