获取数据类型兼容性

const dataViewTag = '[object DataView]';
const mapTag = '[object Map]';
const objectTag = '[object Object]';
const promiseTag = '[object Promise]';
const setTag = '[object Set]';
const weakMapTag = '[object WeakMap]';

/** Used to detect maps, sets, and weakmaps. */
const dataViewCtorString = `${DataView}`;
const mapCtorString = `${Map}`;
const promiseCtorString = `${Promise}`;
const setCtorString = `${Set}`;
const weakMapCtorString = `${WeakMap}`;

let getTag = baseGetTag;

// Fallback for data views, maps, sets, and weak maps in IE 11 and promises in Node.js < 6.
if (
    (DataView && getTag(new DataView(new ArrayBuffer(1))) != dataViewTag)
    || (getTag(new Map) != mapTag)
    || (getTag(Promise.resolve()) != promiseTag)
    || (getTag(new Set) != setTag)
    || (getTag(new WeakMap) != weakMapTag)) {
    // 扩展baseGetTag函数
    getTag = (value) => {
      const result = baseGetTag(value);
      const Ctor = result == objectTag ? value.constructor : undefined;
      const ctorString = Ctor ? `${Ctor}` : '';

      if (ctorString) {
        switch (ctorString) {
          case dataViewCtorString: return dataViewTag;
          case mapCtorString: return mapTag;
          case promiseCtorString: return promiseTag;
          case setCtorString: return setTag;
          case weakMapCtorString: return weakMapTag;
        }
      }
      return result;
   }
}

首先获取了全局构造函数，根据这些构造函数字符来判断当前浏览器是否能够识别ES6新的数据类型。

如果不识别，则获取实例的构造函数，通过toSring和全局构造函数字符对比，从而兼容的获取了数据类型。

Promise.resolve，Promise需要调用resolve才能获取到Promise对象，否则为函数类型。