SOURCE

function convertAddr(source) {
  try {
    const target = source.split('').map(i => {
      const a = 'a'.charCodeAt(0)
      const z = 'z'.charCodeAt(0)
      const one = '1'.charCodeAt(0)
      const five = '5'.charCodeAt(0)
      const charCode = i.charCodeAt(0)
      if (charCode >= a && charCode <= z) {
        return charCode - a + 6
      }
      if (charCode >= one && charCode <= five) {
        return charCode - one + 1
      }
      if (i === '.') {
        return 0
      }
      throw new Error('Address include illegal character')
    }).join('')
    return '0xffffffffff' + target
  } catch (err) {
    return err.message
  }
}

const target = convertAddr('a')
console.log(target)