printWarning = function printWarning(format) {
    for (var _len = arguments.length, args = Array(_len > 1 ? _len - 1 : 0), _key = 1; _key < _len; _key++) {
      args[_key - 1] = arguments[_key];
    }

    var argIndex = 0;
    var message = 'Warning: ' + format.replace(/%s/g, function () {
      return args[argIndex++];
    });
    if (typeof console !== 'undefined') {
      console.error(message);
    }
    try {
      // --- Welcome to debugging React ---
      // This error was thrown as a convenience so that you can use this stack
      // to find the callsite that caused this warning to fire.
      throw new Error(message);
    } catch (x) {}
  }
Invoked when the component is mounted. Values in the mapping will be set on
`this.props` if that prop is not specified (i.e. using an `in` check).

This method is invoked before `getInitialState` and therefore cannot rely
on `this.state` or use `this.setState`.

@return {object}
@optional