_.reduce(collection, [iteratee=_.identity], [accumulator])

84

_.reduce(collection, [iteratee=_.identity], [accumulator])

_.reduce类似原生的reduce将每一个集合中的元素通过iteratee处理后的结果累加到一个值，每一次连续的调用的累加值都是上一次的返回值。如果初始累加值没有提供，就使用第一次循环的返回值

参数

collection (Array|Object): 需要遍历的集合
[iteratee=_.identity] (Function): 每一次遍历元素调用的方法
[accumulator] (*): 初始累加器值

返回值

(*): 返回累加好的值

例子

_.reduce([1, 2], function(sum, n) {
  return sum + n;
}, 0);
// => 3
 
_.reduce({ 'a': 1, 'b': 2, 'c': 1 }, function(result, value, key) {
  (result[value] || (result[value] = [])).push(key);
  return result;
}, {});
// => { '1': ['a', 'c'], '2': ['b'] } (iteration order is not guaranteed)

源代码：

import arrayReduce from './.internal/arrayReduce.js'
import baseEach from './.internal/baseEach.js'
import baseReduce from './.internal/baseReduce.js'

/**
 * Reduces `collection` to a value which is the accumulated result of running
 * each element in `collection` thru `iteratee`, where each successive
 * invocation is supplied the return value of the previous. If `accumulator`
 * is not given, the first element of `collection` is used as the initial
 * value. The iteratee is invoked with four arguments:
 * (accumulator, value, index|key, collection).
 *
 * Many lodash methods are guarded to work as iteratees for methods like
 * `reduce`, `reduceRight`, and `transform`.
 *
 * The guarded methods are:
 * `assign`, `defaults`, `defaultsDeep`, `includes`, `merge`, `orderBy`,
 * and `sortBy`
 *
 * @since 0.1.0
 * @category Collection
 * @param {Array|Object} collection The collection to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @param {*} [accumulator] The initial value.
 * @returns {*} Returns the accumulated value.
 * @see reduceRight, transform
 * @example
 *
 * reduce([1, 2], (sum, n) => sum + n, 0)
 * // => 3
 *
 * reduce({ 'a': 1, 'b': 2, 'c': 1 }, (result, value, key) => {
 *   (result[value] || (result[value] = [])).push(key)
 *   return result
 * }, {})
 * // => { '1': ['a', 'c'], '2': ['b'] } (iteration order is not guaranteed)
 */
//将每一个集合中的元素通过iteratee处理后的结果累加到一个值，每一次连续的调用的累加值都是上一次的返回值。如果初始累加值没有提供，就使用第一次循环的返回值
function reduce(collection, iteratee, accumulator) {
  const func = Array.isArray(collection) ? arrayReduce : baseReduce//判断collection参数是数组还是其他类型，如果是数组调用arrayReduce，否则baseReduce
  const initAccum = arguments.length < 3
  return func(collection, iteratee, accumulator, initAccum, baseEach)
}

export default reduce

arrayReduce

/**
 * A specialized version of `reduce` for arrays.
 *
 * @private
 * @param {Array} [array] The array to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @param {*} [accumulator] The initial value.
 * @param {boolean} [initAccum] Specify using the first element of `array` as
 *  the initial value.
 * @returns {*} Returns the accumulated value.
 */
//为数组类型数据提供的reduce方法
function arrayReduce(array, iteratee, accumulator, initAccum) {
  let index = -1//循环索引
  const length = array == null ? 0 : array.length//数组长度

  if (initAccum && length) {//如果没有提供累加器初始值，且数组至少有一个元素，累加器初始值就是数组第一个元素
    accumulator = array[++index]
  }
  while (++index < length) {//循环累加
    accumulator = iteratee(accumulator, array[index], index, array)
  }
  return accumulator
}

export default arrayReduce

baseReduce

/**
 * The base implementation of `reduce` and `reduceRight` which iterates
 * over `collection` using `eachFunc`.
 *
 * @private
 * @param {Array|Object} collection The collection to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @param {*} accumulator The initial value.
 * @param {boolean} initAccum Specify using the first or last element of
 *  `collection` as the initial value.
 * @param {Function} eachFunc The function to iterate over `collection`.
 * @returns {*} Returns the accumulated value.
 */
//reduce基础实现，使用eachFunc方法执行累加的方法
function baseReduce(collection, iteratee, accumulator, initAccum, eachFunc) {
  eachFunc(collection, (value, index, collection) => {
    accumulator = initAccum
      ? (initAccum = false, value)
      : iteratee(accumulator, value, index, collection)
      //如果没有提供累加初始值，就用第一个元素作为初始值
  })
  return accumulator
}

export default baseReduce

baseEach

import baseForOwn from './baseForOwn.js'
import isArrayLike from '../isArrayLike.js'

/**
 * The base implementation of `forEach`.
 *
 * @private
 * @param {Array|Object} collection The collection to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {Array|Object} Returns `collection`.
 */
//forEach的基础实现
//collection被迭代遍历的对象，iteratee遍历器
function baseEach(collection, iteratee) {
  if (collection == null) {//如果collection为空，直接返回原对象
    return collection
  }
  if (!isArrayLike(collection)) {//如果collection不是array-like对象，就调用baseForOwn处理
    return baseForOwn(collection, iteratee)
  }
  const length = collection.length//对象长度
  const iterable = Object(collection)
  let index = -1//循环索引

  while (++index < length) {//循环调用iteratee方法，如果iteratee返回值是false，提前跳出循环结束
    if (iteratee(iterable[index], index, iterable) === false) {
      break
    }
  }
  return collection
}

export default baseEach

baseForOwn

import baseFor from './baseFor.js'
import keys from '../keys.js'

/**
 * The base implementation of `forOwn`.
 *
 * @private
 * @param {Object} object The object to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {Object} Returns `object`.
 */
//forOwn方法的基础实现
function baseForOwn(object, iteratee) {
  return object && baseFor(object, iteratee, keys)//如果object不为空，调用baseFor处理
}

export default baseForOwn

baseFor

/**
 * The base implementation of `baseForOwn` which iterates over `object`
 * properties returned by `keysFunc` and invokes `iteratee` for each property.
 * Iteratee functions may exit iteration early by explicitly returning `false`.
 *
 * @private
 * @param {Object} object The object to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @param {Function} keysFunc The function to get the keys of `object`.
 * @returns {Object} Returns `object`.
 */
//baseForOwn的基础实现，通过keysFunc遍历对象每一个属性，用iteratee处理每个属性值
//iteratee可以返回false跳出循环
function baseFor(object, iteratee, keysFunc) {
  const iterable = Object(object)
  const props = keysFunc(object)//key组成的数组
  let { length } = props//key数组长度
  let index = -1//循环索引

  while (length--) {//遍历key数组
    const key = props[++index]//当前key
    if (iteratee(iterable[key], key, iterable) === false) {//调用iteratee
      break
    }
  }
  return object
}

export default baseFor

keys

import arrayLikeKeys from './.internal/arrayLikeKeys.js'
import isArrayLike from './isArrayLike.js'

/**
 * Creates an array of the own enumerable property names of `object`.
 *
 * **Note:** Non-object values are coerced to objects. See the
 * [ES spec](http://ecma-international.org/ecma-262/7.0/#sec-object.keys)
 * for more details.
 *
 * @since 0.1.0
 * @category Object
 * @param {Object} object The object to query.
 * @returns {Array} Returns the array of property names.
 * @see values, valuesIn
 * @example
 *
 * function Foo() {
 *   this.a = 1
 *   this.b = 2
 * }
 *
 * Foo.prototype.c = 3
 *
 * keys(new Foo)
 * // => ['a', 'b'] (iteration order is not guaranteed)
 *
 * keys('hi')
 * // => ['0', '1']
 */
//创建一个给定对象的自身可枚举属性组成的数组
function keys(object) {
  return isArrayLike(object)
    ? arrayLikeKeys(object)
    : Object.keys(Object(object))
    //如果object是array-like对象，就调用arrayLikeKeys，否则使用原生的Object.keys方法
}

export default keys

arrayLikeKeys

import isArguments from '../isArguments.js'
import isBuffer from '../isBuffer.js'
import isIndex from './isIndex.js'
import isTypedArray from '../isTypedArray.js'

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

/**
 * Creates an array of the enumerable property names of the array-like `value`.
 *
 * @private
 * @param {*} value The value to query.
 * @param {boolean} inherited Specify returning inherited property names.
 * @returns {Array} Returns the array of property names.
 */
//创建一个给定array-like对象的自身可枚举属性组成的数组
function arrayLikeKeys(value, inherited) {
  const isArr = Array.isArray(value)//是否是数组
  const isArg = !isArr && isArguments(value)//是否是arguments对象
  const isBuff = !isArr && !isArg && isBuffer(value)//是否是buffer对象
  const isType = !isArr && !isArg && !isBuff && isTypedArray(value)//是否是typedArray
  const skipIndexes = isArr || isArg || isBuff || isType//这四种类型需要跳过数字索引
  const length = value.length//value的长度
  const result = new Array(skipIndexes ? length : 0)//结果数组
  let index = skipIndexes ? -1 : length//循环索引
  while (++index < length) {
    result[index] = `${index}`
  }
  for (const key in value) {//循环属性，将非index属性和非特殊属性push进结果数组
    if ((inherited || hasOwnProperty.call(value, key)) &&
        !(skipIndexes && (
           // Safari 9 has enumerable `arguments.length` in strict mode.
           (key == 'length' ||
           // Node.js 0.10 has enumerable non-index properties on buffers.
           (isBuff && (key == 'offset' || key == 'parent')) ||
           // PhantomJS 2 has enumerable non-index properties on typed arrays.
           (isType && (key == 'buffer' || key == 'byteLength' || key == 'byteOffset')) || // Skip index properties.
           isIndex(key, length))
        ))) {
      result.push(key)
    }
  }
  return result
}

export default arrayLikeKeys