js type命令？

Question

看插件源码偶然发现这种写法：
type Axis = 'both' | 'x' | 'y' | 'none';
import type {Element as ReactElement, Node as ReactNode} from 'react';
请问里面的type是什么意思？

import React from 'react';
import PropTypes from 'prop-types';
import {DraggableCore} from 'react-draggable';
import cloneElement from './cloneElement';
import type {Element as ReactElement, Node as ReactNode} from 'react';

type Axis = 'both' | 'x' | 'y' | 'none';
type State = {
  resizing: boolean,
  width: number, height: number,
  slackW: number, slackH: number
};
type DragCallbackData = {
  node: HTMLElement,
  x: number, y: number,
  deltaX: number, deltaY: number,
  lastX: number, lastY: number
};
export type ResizeCallbackData = {
  node: HTMLElement,
  size: {width: number, height: number}
};

//...

Answer 1

这个是flow的类型别名。 更多请查看： https://flow.org/en/docs/type...

Answer 2

据我所知 js 并没有 type 这个东西, 这个应该是 js 之上搞得什么语法