relative.js

// Licensed io.js code
posix.relative = function(from, to) {
  assertPath(from);
  assertPath(to);

  from = posix.resolve(from).substr(1);
  to = posix.resolve(to).substr(1);

  function trim(arr) {
    var start = 0;
    for (; start < arr.length; start++) {
      if (arr[start] !== '') break;
    }

    var end = arr.length - 1;
    for (; end >= 0; end--) {
      if (arr[end] !== '') break;
    }

    if (start > end) return [];
    return arr.slice(start, end + 1);
  }

  var fromParts = trim(from.split('/'));
  var toParts = trim(to.split('/'));

  var length = Math.min(fromParts.length, toParts.length);
  var samePartsLength = length;
  for (var i = 0; i < length; i++) {
    if (fromParts[i] !== toParts[i]) {
      samePartsLength = i;
      break;
    }
  }

  var outputParts = [];
  for (var i = samePartsLength; i < fromParts.length; i++) {
    outputParts.push('..');
  }

  outputParts = outputParts.concat(toParts.slice(samePartsLength));

  return outputParts.join('/');
};