index.js

/**
 * Exports
 */

module.exports = cellref
module.exports.toA1 = convertR1C1toA1
module.exports.toR1C1 = convertA1toR1C1

/**
 * R1C1 pattern
 *
 * @type {RegExp}
 */

const R1C1 = /^R([1-9]\d*)C([1-9]\d*)$/

/**
 * A1 pattern
 *
 * @type {RegExp}
 */

const A1 = /^([A-Z]+)(\d+)$/

/**
 * Auto detect notation used and convert to the opposite notation
 *
 * @param   {string} ref
 * @returns {string}
 * @throws  {Error}
 */

function cellref (ref) {
  if (R1C1.test(ref)) {
    return convertR1C1toA1(ref)
  }

  if (A1.test(ref)) {
    return convertA1toR1C1(ref)
  }

  throw new Error(`could not detect cell reference notation for ${ref}`)
}

/**
 * Convert A1 notation to R1C1 notation
 *
 * @param   {string} ref
 * @returns {string}
 * @throws  {Error}
 */

function convertA1toR1C1 (ref) {
  if (!A1.test(ref)) {
    throw new Error(`${ref} is not a valid A1 cell reference`)
  }

  const refParts = ref
    .replace(A1, '$1,$2')
    .split(',')

  const columnStr = refParts[0]
  const row = refParts[1]
  let column = 0

  for (let i = 0; i < columnStr.length; i++) {
    column = 26 * column + columnStr.charCodeAt(i) - 64
  }

  return `R${row}C${column}`
}

/**
 * Convert R1C1 notation to A1 notation
 *
 * @param {string} ref
 * @returns {string}
 * @throws {Error}
 */

function convertR1C1toA1 (ref) {
  if (!R1C1.test(ref)) {
    throw new Error(`${ref} is not a valid R1C1 cell reference`)
  }

  const refParts = ref
    .replace(R1C1, '$1,$2')
    .split(',')

  const row = refParts[0]
  let column = refParts[1]
  let columnStr = ''

  for (; column; column = Math.floor((column - 1) / 26)) {
    columnStr = String.fromCharCode(((column - 1) % 26) + 65) + columnStr
  }

  return columnStr + row
}