string.split.js

String.prototype.split_ = function (sep, limit = this.length) {
  if (sep === undefined)  return [String(this)]
  if (limit === Infinity)  return []
  
  const parts = [],   length = this.length,   len = sep.length

  const isSepStart = i => {
    for (let j = 0; j < len; j++) {
      if (this[i + j] != sep[j])  return false
    }
    return true
  }

  for (let i = 0;  i <= length && parts.length < limit;  i++) {
    let part = ''

    while (i < length && !isSepStart(i))   part += this[i++]

    if (len)  i += len - 1
    else  part = this[i]

    if (part !== undefined)  parts[parts.length] = part
  }

  return parts
}

results = ([
  {
    str: 'a bc def',
    split: 'a bc def'.split(),
    split_: 'a bc def'.split_(),
  },
  {
    str: 'a bc def', substr: ' ',
    split: 'a bc def'.split(' '),
    split_: 'a bc def'.split_(' '),
  },
  {
    str: '  a  bc    def  ', substr: '  ',
    split: '  a  bc    def  '.split('  '),
    split_: '  a  bc    def  '.split_('  '),
  },
  {
    str: 'a - bc - def', substr: ' - ',
    split: 'a - bc - def'.split(' - '),
    split_: 'a - bc - def'.split_(' - '),
  },
  {
    str: 'a bc def', substr: '',
    split: 'a bc def'.split(''),
    split_: 'a bc def'.split_(''),
  },
  {
    str: 'a bc def', substr: '', limit: 1,
    split: 'a bc def'.split('', 1),
    split_: 'a bc def'.split_('', 1),
  },
  {
    str: 'a bc def', substr: '', limit: 4,
    split: 'a bc def'.split('', 4),
    split_: 'a bc def'.split_('', 4),
  },
  {
    str: 'a bc def', substr: '', limit: 99,
    split: 'a bc def'.split('', 99),
    split_: 'a bc def'.split_('', 99),
  },
  {
    str: 'a bc def', substr: '', limit: Infinity,
    split: 'a bc def'.split('', Infinity),
    split_: 'a bc def'.split_('', Infinity),
  },
])