PHP's addcslashes in JavaScript

module.exports = functionaddcslashes (str, charlist) {
//  discuss at: https://locutus.io/php/addcslashes/
// original by: Brett Zamir (https://brett-zamir.me)
//      note 1: We show double backslashes in the return value example
//      note 1: code below because a JavaScript string will not
//      note 1: render them as backslashes otherwise
//   example 1: addcslashes('foo[ ]', 'A..z'); // Escape all ASCII within capital A to lower z range, including square brackets
//   returns 1: "\\f\\o\\o\\[ \\]"
//   example 2: addcslashes("zoo['.']", 'z..A'); // Only escape z, period, and A here since not a lower-to-higher range
//   returns 2: "\\zoo['\\.']"
//   _example 3: addcslashes("@a\u0000\u0010\u00A9", "\0..\37!@\177..\377"); // Escape as octals those specified and less than 32 (0x20) or greater than 126 (0x7E), but not otherwise
//   _returns 3: '\\@a\\000\\020\\302\\251'
//   _example 4: addcslashes("\u0020\u007E", "\40..\175"); // Those between 32 (0x20 or 040) and 126 (0x7E or 0176) decimal value will be backslashed if specified (not octalized)
//   _returns 4: '\\ ~'
//   _example 5: addcslashes("\r\u0007\n", '\0..\37'); // Recognize C escape sequences if specified
//   _returns 5: "\\r\\a\\n"
//   _example 6: addcslashes("\r\u0007\n", '\0'); // Do not recognize C escape sequences if not specified
//   _returns 6: "\r\u0007\n"
let target = ''
const chrs = []
let i = 0
let j = 0
let c = ''
let next = ''
let rangeBegin = ''
let rangeEnd = ''
let chr = ''
let begin = 0
let end = 0
let octalLength = 0
let postOctalPos = 0
let cca = 0
let escHexGrp = []
let encoded = ''
const percentHex = /%([\dA-Fa-f]+)/g
const _pad = function (n, c) {
if ((n = n + '').length < c) {
returnnewArray(++c - n.length).join('0') + n
    }
return n
  }
for (i = 0; i < charlist.length; i++) {
    c = charlist.charAt(i)
    next = charlist.charAt(i + 1)
if (c === '\\' && next && (/\d/).test(next)) {
// Octal
      rangeBegin = charlist.slice(i + 1).match(/^\d+/)[0]
      octalLength = rangeBegin.length
      postOctalPos = i + octalLength + 1
if (charlist.charAt(postOctalPos) + charlist.charAt(postOctalPos + 1) === '..') {
// Octal begins range
        begin = rangeBegin.charCodeAt(0)
if ((/\\\d/).test(charlist.charAt(postOctalPos + 2) + charlist.charAt(postOctalPos + 3))) {
// Range ends with octal
          rangeEnd = charlist.slice(postOctalPos + 3).match(/^\d+/)[0]
// Skip range end backslash
          i += 1
        } elseif (charlist.charAt(postOctalPos + 2)) {
// Range ends with character
          rangeEnd = charlist.charAt(postOctalPos + 2)
        } else {
thrownewError('Range with no end point')
        }
        end = rangeEnd.charCodeAt(0)
if (end > begin) {
// Treat as a range
for (j = begin; j <= end; j++) {
            chrs.push(String.fromCharCode(j))
          }
        } else {
// Supposed to treat period, begin and end as individual characters only, not a range
          chrs.push('.', rangeBegin, rangeEnd)
        }
// Skip dots and range end (already skipped range end backslash if present)
        i += rangeEnd.length + 2
      } else {
// Octal is by itself
        chr = String.fromCharCode(parseInt(rangeBegin, 8))
        chrs.push(chr)
      }
// Skip range begin
      i += octalLength
    } elseif (next + charlist.charAt(i + 2) === '..') {
// Character begins range
      rangeBegin = c
      begin = rangeBegin.charCodeAt(0)
if ((/\\\d/).test(charlist.charAt(i + 3) + charlist.charAt(i + 4))) {
// Range ends with octal
        rangeEnd = charlist.slice(i + 4).match(/^\d+/)[0]
// Skip range end backslash
        i += 1
      } elseif (charlist.charAt(i + 3)) {
// Range ends with character
        rangeEnd = charlist.charAt(i + 3)
      } else {
thrownewError('Range with no end point')
      }
      end = rangeEnd.charCodeAt(0)
if (end > begin) {
// Treat as a range
for (j = begin; j <= end; j++) {
          chrs.push(String.fromCharCode(j))
        }
      } else {
// Supposed to treat period, begin and end as individual characters only, not a range
        chrs.push('.', rangeBegin, rangeEnd)
      }
// Skip dots and range end (already skipped range end backslash if present)
      i += rangeEnd.length + 2
    } else {
// Character is by itself
      chrs.push(c)
    }
  }
for (i = 0; i < str.length; i++) {
    c = str.charAt(i)
if (chrs.indexOf(c) !== -1) {
      target += '\\'
      cca = c.charCodeAt(0)
if (cca < 32 || cca > 126) {
// Needs special escaping
switch (c) {
case'\n':
            target += 'n'
break
case'\t':
            target += 't'
break
case'\u000D':
            target += 'r'
break
case'\u0007':
            target += 'a'
break
case'\v':
            target += 'v'
break
case'\b':
            target += 'b'
break
case'\f':
            target += 'f'
break
default:
// target += _pad(cca.toString(8), 3);break; // Sufficient for UTF-16
            encoded = encodeURIComponent(c)
// 3-length-padded UTF-8 octets
if ((escHexGrp = percentHex.exec(encoded)) !== null) {
// already added a slash above:
              target += _pad(parseInt(escHexGrp[1], 16).toString(8), 3)
            }
while ((escHexGrp = percentHex.exec(encoded)) !== null) {
              target += '\\' + _pad(parseInt(escHexGrp[1], 16).toString(8), 3)
            }
break
        }
      } else {
// Perform regular backslashed escaping
        target += c
      }
    } else {
// Just add the character unescaped
      target += c
    }
  }
return target
}