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

const {
  multiplyCurrencies,
  subtractCurrencies,
} = require('../../../../../helpers/utils/conversion-util')
const ethUtil = require('ethereumjs-util')

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

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

module.exports = {
  calcMaxAmount,
}