# lodash 源码阅读-baseClone

/**
 * The base implementation of `clone` and `cloneDeep` which tracks
 * traversed objects.
 *
 * @private
 * @param {*} value The value to clone. <-- 要克隆的值
 * @param {number} bitmask The bitmask flags.
 *  1 - Deep clone
 *  2 - Flatten inherited properties
 *  4 - Clone symbols
 * @param {Function} [customizer] The function to customize cloning. <-- 自定义克隆的方法
 * @param {string} [key] The key of `value`. <-- value 的 key
 * @param {Object} [object] The parent object of `value`. <-- value 的父级对象
 * @param {Object} [stack] Tracks traversed objects and their clone counterparts. <-- 追踪遍历的对象以及克隆的副本
 * @returns {*} Returns the cloned value.
 */
function baseClone(value, bitmask, customizer, key, object, stack) {
  let result
  const isDeep = bitmask & CLONE_DEEP_FLAG // & 是 位运算符 见下面补充
  const isFlat = bitmask & CLONE_FLAT_FLAG
  const isFull = bitmask & CLONE_SYMBOLS_FLAG

  // 如果有自定义的方法，调用自定义方法
  if (customizer) {
    result = object ? customizer(value, key, object, stack) : customizer(value)
  }
  if (result !== undefined) {
    return result
  }
  // 判断是不是对象
  if (!isObject(value)) {
    return value
  }
  // 判断是不是数组
  const isArr = Array.isArray(value)
  // 返回对象的 toString() 后的值，比如 '[object Object]'
  const tag = getTag(value)
  // 如果是数组
  if (isArr) {
    result = initCloneArray(value) // 初始化一个和 value 长度一样，但是值都是 empty 的数组
    if (!isDeep) {
      return copyArray(value, result) // 把 value 的 值 拷贝到 result 上
    }
  }
  // 如果不是数组
  else {
    const isFunc = typeof value == 'function' // 是否是 function
    // Buffer 参考 NodeJS
    if (isBuffer(value)) {
      return cloneBuffer(value, isDeep)
    }
    // 如果是 对象 || arguments || function 且没有父级。tag 类型参考下面
    if (tag == objectTag || tag == argsTag || (isFunc && !object)) {
      // initCloneObject 拷贝 value 的原型对象
      result = (isFlat || isFunc) ? {} : initCloneObject(value)
      if (!isDeep) {
        return isFlat
        // keysIn 返回 value 自身的以及继承的可枚举属性
        // copyObject <-- 将 value 中包含 keysIn 返回的属性拷贝到 result 上
        // copySymbolsIn 将 value 的自身和继承的 symbol 属性拷贝给 result
          ? copySymbolsIn(value, copyObject(value, keysIn(value), result))
          // Object.assign(result, value) 将 value 的可枚举属性拷贝给 result
          // copySymbols 将 value 的 symbol 属性拷贝给 result
          : copySymbols(value, Object.assign(result, value))
      }
    }
    else {
      // cloneableTags 参考下面
      if (isFunc || !cloneableTags[tag]) {
        return object ? value : {}
      }
      // initCloneByTag 见下面
      result = initCloneByTag(value, tag, isDeep)
    }
  }
  // Check for circular references and return its corresponding clone.
  stack || (stack = new Stack)
  const stacked = stack.get(value)
  if (stacked) {
    return stacked
  }
  stack.set(value, result)

  if (tag == mapTag) {
    value.forEach((subValue, key) => {
      result.set(key, baseClone(subValue, bitmask, customizer, key, value, stack))
    })
    return result
  }

  if (tag == setTag) {
    value.forEach((subValue) => {
      result.add(baseClone(subValue, bitmask, customizer, subValue, value, stack))
    })
    return result
  }

  if (isTypedArray(value)) {
    return result
  }

  const keysFunc = isFull
    ? (isFlat ? getAllKeysIn : getAllKeys)
    : (isFlat ? keysIn : keys)

  const props = isArr ? undefined : keysFunc(value)
  arrayEach(props || value, (subValue, key) => {
    if (props) {
      key = subValue
      subValue = value[key]
    }
    // Recursively populate clone (susceptible to call stack limits).
    assignValue(result, key, baseClone(subValue, bitmask, customizer, key, value, stack))
  })
  return result
}

a & b;
// 在a,b的位表示中，每一个对应的位都为1则返回1， 否则返回0.

/** Used to check objects for own properties. */
const hasOwnProperty = Object.prototype.hasOwnProperty

/**
 * Initializes an array clone.
 *
 * @private
 * @param {Array} array The array to clone.
 * @returns {Array} Returns the initialized clone.
 */
function initCloneArray(array) {
  const { length } = array
  const result = new array.constructor(length) // 调用 array 的构造函数，创建一个新数组

  // Add properties assigned by `RegExp#exec`. 增加正则 exec 后返回的数组的属性
  /**
   * 比如
   * /a/.exec('abc');
   * ["a", index: 0, input: "abc", groups: undefined, length: 1]
   */
  if (length && typeof array[0] == 'string' && hasOwnProperty.call(array, 'index')) {
    result.index = array.index
    result.input = array.input
  }
  return result
}

const hasOwnProperty = Object.prototype.hasOwnProperty;
const arr = ['a', 'b'];
hasOwnProperty.call(arr, 'index'); // false

/** `Object#toString` result references. */
const argsTag = '[object Arguments]'
const arrayTag = '[object Array]'
const boolTag = '[object Boolean]'
const dateTag = '[object Date]'
const errorTag = '[object Error]'
const mapTag = '[object Map]'
const numberTag = '[object Number]'
const objectTag = '[object Object]'
const regexpTag = '[object RegExp]'
const setTag = '[object Set]'
const stringTag = '[object String]'
const symbolTag = '[object Symbol]'
const weakMapTag = '[object WeakMap]'

const arrayBufferTag = '[object ArrayBuffer]'
const dataViewTag = '[object DataView]'
const float32Tag = '[object Float32Array]'
const float64Tag = '[object Float64Array]'
const int8Tag = '[object Int8Array]'
const int16Tag = '[object Int16Array]'
const int32Tag = '[object Int32Array]'
const uint8Tag = '[object Uint8Array]'
const uint8ClampedTag = '[object Uint8ClampedArray]'
const uint16Tag = '[object Uint16Array]'
const uint32Tag = '[object Uint32Array]'

/** Used to identify `toStringTag` values supported by `clone`. */
const cloneableTags = {}
cloneableTags[argsTag] = cloneableTags[arrayTag] =
cloneableTags[arrayBufferTag] = cloneableTags[dataViewTag] =
cloneableTags[boolTag] = cloneableTags[dateTag] =
cloneableTags[float32Tag] = cloneableTags[float64Tag] =
cloneableTags[int8Tag] = cloneableTags[int16Tag] =
cloneableTags[int32Tag] = cloneableTags[mapTag] =
cloneableTags[numberTag] = cloneableTags[objectTag] =
cloneableTags[regexpTag] = cloneableTags[setTag] =
cloneableTags[stringTag] = cloneableTags[symbolTag] =
cloneableTags[uint8Tag] = cloneableTags[uint8ClampedTag] =
cloneableTags[uint16Tag] = cloneableTags[uint32Tag] = true
cloneableTags[errorTag] = cloneableTags[weakMapTag] = false

/**
 * Initializes an object clone based on its `toStringTag`.
 *
 * **Note:** This function only supports cloning values with tags of
 * `Boolean`, `Date`, `Error`, `Map`, `Number`, `RegExp`, `Set`, or `String`.
 *
 * @private
 * @param {Object} object The object to clone.
 * @param {string} tag The `toStringTag` of the object to clone.
 * @param {boolean} [isDeep] Specify a deep clone.
 * @returns {Object} Returns the initialized clone.
 */
function initCloneByTag(object, tag, isDeep) {
  const Ctor = object.constructor
  switch (tag) {
    case arrayBufferTag:
      return cloneArrayBuffer(object)

    case boolTag:
    case dateTag:
      return new Ctor(+object)

    case dataViewTag:
      return cloneDataView(object, isDeep)

    case float32Tag: case float64Tag:
    case int8Tag: case int16Tag: case int32Tag:
    case uint8Tag: case uint8ClampedTag: case uint16Tag: case uint32Tag:
      return cloneTypedArray(object, isDeep)

    case mapTag:
      return new Ctor

    case numberTag:
    case stringTag:
      return new Ctor(object)

    case regexpTag:
      // 调用 regexp.constructor 方法克隆
      return cloneRegExp(object)

    case setTag:
      return new Ctor

    case symbolTag:
      // 调用 Symbol.protoype.valueOf 方法克隆
      return cloneSymbol(object)
  }
}