code: function relative(from, to) {
      from = resolve(from).substr(1);
      to = resolve(to).substr(1);
  
      var fromParts = trimArray(from.split('/'));
      var toParts = trimArray(to.split('/'));
  
      var length = Math.min(fromParts.length, toParts.length);
      var samePartsLength = length;
  
      for (var i = 0; i < length; i += 1) {
        if (fromParts[i] !== toParts[i]) {
          samePartsLength = i;
          break;
        }
      }
  
      var outputParts = [];
      for (var i = samePartsLength; i < fromParts.length; i += 1) {
        outputParts.push('..');
      }
  
      outputParts = outputParts.concat(toParts.slice(samePartsLength));
  
      return outputParts.join('/');
    }
docs: make the relative path
  @param {String} from
  @param {String} to