09.精读lodash的cloneDeep方法源码(二).md

前言

书接上篇，继续对cloneDeep方法进行解读。

源码实现

/**
 * The base implementation of `clone` and `cloneDeep` which tracks
 * traversed objects.
 *
 * @private
 * @param {*} value The value to clone.
 * @param {number} bitmask The bitmask flags.
 *  1 - Deep clone
 *  2 - Flatten inherited properties
 *  4 - Clone symbols
 * @param {Function} [customizer] The function to customize cloning.
 * @param {string} [key] The key of `value`.
 * @param {Object} [object] The parent object of `value`.
 * @param {Object} [stack] Tracks traversed objects and their clone counterparts.
 * @returns {*} Returns the cloned value.
 */
function baseClone(value, bitmask, customizer, key, object, stack) {
    let result
    const isDeep = bitmask & CLONE_DEEP_FLAG
    const isFlat = bitmask & CLONE_FLAT_FLAG
    const isFull = bitmask & CLONE_SYMBOLS_FLAG

    if (customizer) {
        result = object ? customizer(value, key, object, stack) : customizer(value)
    }
    if (result !== undefined) {
        return result
    }
    if (!isObject(value)) {
        return value
    }
    const isArr = Array.isArray(value)
    const tag = getTag(value)
    if (isArr) {
        result = initCloneArray(value)
        if (!isDeep) {
            return copyArray(value, result)
        }
    } else {
        const isFunc = typeof value == 'function'

        if (isBuffer(value)) {
            return cloneBuffer(value, isDeep)
        }
        if (tag == objectTag || tag == argsTag || (isFunc && !object)) {
            result = (isFlat || isFunc) ? {} : initCloneObject(value)
            if (!isDeep) {
                return isFlat
                    ? copySymbolsIn(value, copyObject(value, keysIn(value), result))
                    : copySymbols(value, Object.assign(result, value))
            }
        } else {
            if (isFunc || !cloneableTags[tag]) {
                return object ? value : {}
            }
            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
}

处理对象和函数

if (isBuffer(value)) {
     return cloneBuffer(value, isDeep)
}
function cloneBuffer(buffer, isDeep) {
  if (isDeep) {
    return buffer.slice()
  }
  const length = buffer.length
  const result = allocUnsafe ? allocUnsafe(length) : new buffer.constructor(length)

  buffer.copy(result)
  return result
}

如果在node环境下，通过slice对Buffer拷贝。

if (tag == objectTag || tag == argsTag || (isFunc && !object)) {
    result = (isFlat || isFunc) ? {} : initCloneObject(value)
} 
/**
 * Initializes an object clone.
 *
 * @private
 * @param {Object} object The object to clone.
 * @returns {Object} Returns the initialized clone.
 */
function initCloneObject(object) {
  return (typeof object.constructor == 'function' && !isPrototype(object))
    ? Object.create(Object.getPrototypeOf(object))
    : {}
}

（未完待续...）