_.flatten(array)

15

_.flatten(array)

_.flatten方法将数组展开一层

参数

array (Array): 需要展开的数组

返回值

(Array):返回展开后的新数组

例子

_.flatten([1, [2, [3, [4]], 5]]);
// => [1, 2, [3, [4]], 5]

源代码：

以下是flatten

import baseFlatten from './.internal/baseFlatten.js'

/**
 * Flattens `array` a single level deep.
 *
 * @since 0.1.0
 * @category Array
 * @param {Array} array The array to flatten.
 * @returns {Array} Returns the new flattened array.
 * @see flatMap, flatMapDeep, flatMapDepth, flattenDeep, flattenDepth
 * @example
 *
 * flatten([1, [2, [3, [4]], 5]])
 * // => [1, 2, [3, [4]], 5]
 */
//展开数组一层
function flatten(array) {
  const length = array == null ? 0 : array.length//数组长度
  return length ? baseFlatten(array, 1) : []//如果数组长度为0返回空数组，否则调用baseFlatten展开数组一层
}

export default flatten

下面是baseFlatten

import isFlattenable from './isFlattenable.js'

/**
 * The base implementation of `flatten` with support for restricting flattening.
 *
 * @private
 * @param {Array} array The array to flatten.
 * @param {number} depth The maximum recursion depth.
 * @param {boolean} [predicate=isFlattenable] The function invoked per iteration.
 * @param {boolean} [isStrict] Restrict to values that pass `predicate` checks.
 * @param {Array} [result=[]] The initial result value.
 * @returns {Array} Returns the new flattened array.
 */
//这个方法用于数组展开
//array需要展开操作的数组，depth需要展开的层数
//predicate用于判断值是否可展开
//isStrict标识用于判断是否约束值必须通过predicate方法的检查
function baseFlatten(array, depth, predicate, isStrict, result) {
  predicate || (predicate = isFlattenable)//predicate每次循环都会调用，用来判断当前值是否是一个可展开的array-like对象
  result || (result = [])

  if (array == null) {//需要展开的数组是空，就返回空数组
    return result
  }

  for (const value of array) {
    if (depth > 0 && predicate(value)) {//如果展开层数大于0且当前循环值可展开
      if (depth > 1) {//如果展开层数大于一层就继续递归调用，层数减一
        // Recursively flatten arrays (susceptible to call stack limits).
        baseFlatten(value, depth - 1, predicate, isStrict, result)
      } else {//如果只展开一层，就展开后push到result里
        result.push(...value)
      }
    } else if (!isStrict) {//如果没有传递isStrict标识，就直接将当前循环值push入结果数组
      result[result.length] = value
    }
  }
  return result
}

export default baseFlatten

下面是isFlattenable

import isArguments from '../isArguments.js'

/** Built-in value reference. */
const spreadableSymbol = Symbol.isConcatSpreadable

/**
 * Checks if `value` is a flattenable `arguments` object or array.
 *
 * @private
 * @param {*} value The value to check.
 * @returns {boolean} Returns `true` if `value` is flattenable, else `false`.
 */
//检查一个变量是否是一个可展开的对象或者数组
function isFlattenable(value) {
  return Array.isArray(value) || isArguments(value) ||
    !!(spreadableSymbol && value && value[spreadableSymbol])
    //如果是数组，则可展开
    //如果是arguments对象，则可展开
    //如果当前环境含有Symbol对象，且此变量含有Symbol.isConcatSpreadable属性，Symbol.isConcatSpreadable用于改变array或者array-like对象使用concat时的默认行为
}

export default isFlattenable