metamask-extension/ui/app/pages/confirm-approve/confirm-approve.util.js

import { TRANSACTION_CATEGORIES } from '../../../../shared/constants/transaction';
import { decimalToHex } from '../../helpers/utils/conversions.util';
import {
  calcTokenValue,
  getTokenAddressParam,
} from '../../helpers/utils/token-util';
import { getTokenData } from '../../helpers/utils/transactions.util';

export function getCustomTxParamsData(
  data,
  { customPermissionAmount, decimals },
) {
  const tokenData = getTokenData(data);

  if (!tokenData) {
    throw new Error('Invalid data');
  } else if (tokenData.name !== TRANSACTION_CATEGORIES.TOKEN_METHOD_APPROVE) {
    throw new Error(
      `Invalid data; should be 'approve' method, but instead is '${tokenData.name}'`,
    );
  }
  let spender = getTokenAddressParam(tokenData);
  if (spender.startsWith('0x')) {
    spender = spender.substring(2);
  }
  const [signature, tokenValue] = data.split(spender);

  if (!signature || !tokenValue) {
    throw new Error('Invalid data');
  } else if (tokenValue.length !== 64) {
    throw new Error(
      'Invalid token value; should be exactly 64 hex digits long (u256)',
    );
  }

  let customPermissionValue = decimalToHex(
    calcTokenValue(customPermissionAmount, decimals),
  );
  if (customPermissionValue.length > 64) {
    throw new Error('Custom value is larger than u256');
  }

  customPermissionValue = customPermissionValue.padStart(
    tokenValue.length,
    '0',
  );
  const customTxParamsData = `${signature}${spender}${customPermissionValue}`;
  return customTxParamsData;
}