摘要：寫了幾篇筆記以后發現很多函數大量依賴了內部的基礎函數，一次性讀完有點麻煩，所以還是決定從基礎函數沒有或者很少依賴其他函數看起。

百忙之中（閑來無事）想抽點時間好好讀一下源碼，于是就選了Lodash來寫一個系列罷。寫了幾篇筆記以后發現很多函數大量依賴了內部的基礎函數，一次性讀完有點麻煩，所以還是決定從基礎函數（沒有或者很少依賴其他函數）看起。

文檔地址：中文文檔?? 英文文檔
源碼地址：gayhub

baseGetTag，判斷變量類型。因為typeof在面對new Number(1)這種情況時力有不逮，所以lodash重寫了類型判斷。 javascript秘密花園中也提到了JavaScript 標準庫 推薦的是Object.prototype.toString.call(value)這種方式來判斷類型，不過lodash明顯做的更多，前半段是推薦的方式，后半段目測是針對Symbol類型做了優化，具體步驟待研究，有了解的同學歡迎告訴我。

const objectProto = Object.prototype
const hasOwnProperty = objectProto.hasOwnProperty
const toString = objectProto.toString
const symToStringTag = typeof Symbol != "undefined"
    ? Symbol.toStringTag
    : undefined

function baseGetTag(value) {
    if (value == null) {
        return value === undefined
            ? "[object Undefined]"
            : "[object Null]"
    }
    if (!(symToStringTag && symToStringTag in Object(value))) {
        return toString.call(value)
    }
    const isOwn = hasOwnProperty.call(value, symToStringTag)
    const tag = value[symToStringTag]
    let unmasked = false
    try {
        value[symToStringTag] = undefined
        unmasked = true
    } catch (e) {}

    const result = toString.call(value)
    if (unmasked) {
        if (isOwn) {
            value[symToStringTag] = tag
        } else {
            delete value[symToStringTag]
        }
    }
    return result
}

export default baseGetTag

getTag是在baseGetTag基礎上的包裝，主要是為了兼容IE 11中的 data views, maps, sets, weak maps 和當Node.js < 6時的promises，這一點在源碼注釋中已有體現

import baseGetTag from "./baseGetTag.js"

/** `Object#toString` result references. */
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)) {
    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
    }
}

export default getTag

isObjectLike， 判斷是否是一個對象

function isObjectLike(value) {
  return typeof value == "object" && value !== null
}
export default isObjectLike

isArguments調用了getTag和isObjectLike用來判斷參數是否是一個Arguments對象。

import getTag from "./.internal/getTag.js"
import isObjectLike from "./isObjectLike"
            
function isArguments(value) {
  return isObjectLike(value) && getTag(value) == "[object Arguments]"
}
            
export default isArguments

isFlattenable調用isArguments來判斷是否為一個Arguments對象或數組或是一個能展開的Symbol

import isArguments from "../isArguments.js"

const spreadableSymbol = Symbol.isConcatSpreadable

function isFlattenable(value) {
  return Array.isArray(value) || isArguments(value) ||
    !!(spreadableSymbol && value && value[spreadableSymbol])
}

export default isFlattenable

baseFlatten可以展開數組的n層嵌套（扁平處理），依賴了isFlattenable

    // 例：
    baseFlatten([[[1,2,3],[2,2,3]],4,5,6], 2) // => [1, 2, 3, 2, 2, 3, 4, 5, 6]
    // isStrict == true
    baseFlatten([[1,2,3],[2,2,3],4,5,6], 1, null, true) // => [1, 2, 3, 2, 2, 3]

import isFlattenable from "./isFlattenable.js"

function baseFlatten(array, depth, predicate, isStrict, result) {
  predicate || (predicate = isFlattenable) // predicate參數兜底
  result || (result = []) // 返回新數組

  if (array == null) {
    return result
  }

    // 遞歸調用本身，嚴格模式`isStrict = true`直接退出循環
  for (const value of array) {
    if (depth > 0 && predicate(value)) {
      if (depth > 1) {
        // Recursively flatten arrays (susceptible to call stack limits).
        baseFlatten(value, depth - 1, predicate, isStrict, result)
      } else {
        result.push(...value)
      }
    } else if (!isStrict) {
      result[result.length] = value
    }
  }
  return result
}

export default baseFlatten

isLength，判斷是否是一個有效數字

const MAX_SAFE_INTEGER = 9007199254740991 // 最大數
function isLength(value) {
  // type是number并且大于等于0且小于等于最大數
  return typeof value == "number" &&
    value > -1 && value % 1 == 0 && value <= MAX_SAFE_INTEGER
}

export default isLength

isArrayLike， 判斷是否是一個數組，依賴了isLength

import isLength from "./isLength.js"

function isArrayLike(value) {
   // value不為空，不是function且有length屬性
  return value != null && typeof value != "function" && isLength(value.length)
}

export default isArrayLike

isArrayLikeObject，判斷是否是一個數組或者對象，依賴了isArrayLike和isObjectLike

import isArrayLike from "./isArrayLike.js"
import isObjectLike from "./isObjectLike.js"

function isArrayLikeObject(value) {
  return isObjectLike(value) && isArrayLike(value)
}

export default isArrayLikeObject

eq，判斷兩個值是否相等。

function eq(value, other) {
  return value === other || (value !== value && other !== other)
}

export default eq

||前面好理解，判斷兩個值全等
||后面是為了判斷 NaN

例：

const object = { "a": 1 }
const other = { "a": 1 }

eq(object, object)
// => true

eq(object, other)
// => false

eq("a", "a")
// => true

eq("a", Object("a"))
// => false

eq(NaN, NaN)
// => true

獲取鍵值對數組中的“鍵”的索引。例:

assocIndexOf([[1,2],[3,4],[5,6]], 2) // => -1
assocIndexOf([[1,2],[3,4],[5,6]], 3) // => 1

import eq from "../eq.js"

function assocIndexOf(array, key) {
  let { length } = array
  while (length--) {
    if (eq(array[length][0], key)) {
      return length
    }
  }
  return -1
}

export default assocIndexOf