code: function indexKeys(target, pred) {
    pred = pred || has;
    var results = [];
  
    for (var i = 0, len = target.length; i < len; i += 1) {
      if (pred(target, i)) {
        results.push(String(i));
      }
    }
  
    return results;
  }
docs: indexKeys
  
  @name indexKeys
  @api private
  @param {} target
  @param {} pred
  @return {Array}