lodash中判斷數據類型的方法

lodash庫中判斷數據類型涉及兩個方法，getTag和baseGetTag，下面是baseGetTag：

const objectProto = Object.prototype
const hasOwnProperty = objectProto.hasOwnProperty//Object.prototype.hasOwnProperty
const toString = objectProto.toString//Object.prototype.toString
const symToStringTag = typeof Symbol != 'undefined' ? Symbol.toStringTag : undefined
//當前環境是否支持Symbol然后獲取對應的Symbol.toStringTag

/**
 * The base implementation of `getTag` without fallbacks for buggy environments.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
function baseGetTag(value) {
  if (value == null) {//如果值是undefined和null返回對應tag
    return value === undefined ? '[object Undefined]' : '[object Null]'
  }
  if (!(symToStringTag && symToStringTag in Object(value))) {
    //如果不支持Symbol或者value值上面沒有Symbol.toStringTag屬性，直接返回Object.prototype.toString調用后的值
    return toString.call(value)
  }
  const isOwn = hasOwnProperty.call(value, symToStringTag)//value自身是否有Symbol.toStringTag屬性
  const tag = value[symToStringTag]//存下value上的Symbol.toStringTag屬性
  let unmasked = false//將value自身上的Symbol.toStringTag屬性重置為undefined是否成功
  try {//嘗試將value自身上的Symbol.toStringTag屬性重置為空
    value[symToStringTag] = undefined
    unmasked = true
  } catch (e) {}

  const result = toString.call(value)
  //value自身Symbol.toStringTag屬性置空后獲取toString處理后的tag
  if (unmasked) {//如果Symbol.toStringTag屬性置空成功，再將Symbol.toStringTag屬性變回原樣
    if (isOwn) {
      value[symToStringTag] = tag
    } else {
      delete value[symToStringTag]
    }
  }
  return result
}

export default baseGetTag

下面是getTag：

import baseGetTag from './baseGetTag.js'

/** `Object#toString` result references. */
//Dateview，Map，Promise，Set，WeakMap這幾種類型和Object應該返回的類型tag
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. */
//Dateview，Map，Promise，Set，WeakMap這幾種類型的構造函數屬性返回的字符串
const dataViewCtorString = `${DataView}`
const mapCtorString = `${Map}`
const promiseCtorString = `${Promise}`
const setCtorString = `${Set}`
const weakMapCtorString = `${WeakMap}`

/**
 * Gets the `toStringTag` of `value`.
 *
 * @private
 * @param {*} value The value to query.
 * @returns {string} Returns the `toStringTag`.
 */
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)) {//如果在某些環境下無法判斷這5種類型，就做一下兼容處理
  getTag = (value) => {
    const result = baseGetTag(value)
    const Ctor = result == objectTag ? value.constructor : undefined
    const ctorString = Ctor ? `${Ctor}` : ''
    //獲取值的constructor構造函數屬性，如果獲取到了就返回對應的tag，如果獲取不到就返回原始toString值
    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
  }
}

export default getTag