proper-slice.js

function slice(str, start, end) {
  let i = 0, pos = 0, legacyStart, legacyEnd;
  for (let ch of str) {
    if (i === start) {
      legacyStart = pos;
    }
    i++;
    pos += ch.length;
    if (i === end) {
      legacyEnd = pos;
      break;
    }
  }
  return str.slice(legacyStart, legacyEnd);
}