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

import ethUtil from 'ethereumjs-util'
import {
  multiplyCurrencies,
  subtractCurrencies,
} from '../../../../../helpers/utils/conversion-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(
        ethUtil.addHexPrefix(balance),
        ethUtil.addHexPrefix(gasTotal),
        { toNumericBase: 'hex' },
      )
}