A, filter

filter array for filtering.
It creates a new array of new elements in the array by checking the specified array qualifying all the elements.

Note: filter () will not be an empty array detection.

Note: filter () does not alter the original array.

1. Grammar

Array.filter(function(currentValue, indedx, arr), thisValue)

The first parameter is a function of

This callback function's return value is a boolean value

Will return true when checking elements meet the filter criteria, the internal function will automatically add a new element to this array

When not meet the conditions will return false, the internal function will filter out this element

Example 1 : Returns an array of elements is less than 100

const nums = [10, 20, 50, 101, 222,40]
  let newNums = nums.filter(function (n) {
    return n < 100
  })
  console.log(newNums); //[10, 20, 50, 40]

Example 2 : Screening of Space

const arr = ['0',1,2,3,4,"",5,1,4,'0',""];
  let arr_filter = arr.filter(function(x){
    return x;/* 筛选空格 */
  })
  console.log(arr_filter)

Example 3 : removal Arrayof repetitive elements

const arr_repeat = ['A', 'B', 'A', 'B', 'B', 'C', 'A', 'D', 'C']
  let arr_filter = arr_repeat.filter(function (el, index, self) {
//    console.log(self.indexOf(el))
//    console.log('index',index)
    return self.indexOf(el) == index

  })

console.log(arr_filter) //["A", "B", "C", "D"]

Remove duplicate elements relied indexOfalways returns the position of an element of the first occurrence of subsequent repetitive elements position index and indexOfunequal position return, so it is filterfiltered out.

Two, map

map () method: Each element in the original array after a specified method call returns an array of new values returns.

const nums = [2, 4, 6, 8, 10]
  newNums = nums.map(function (n) {
    return n*2
  })
  console.log(newNums)

Three, reduce

reduce the function of all the contents of the array are aggregated.

reduce()To a function in this role Arrayis [x1, x2, x3...]on, this function must receive two parameters, reduce()the results and continues to the next element in the sequence do cumulative basis, the effect is:

[x1, x2, x3, x4].reduce(f) = f(f(f(x1, x2), x3), x4)

Syntax :

arr.reduce(callback,[initialValue])

The first parameter is the callback function

The callback function takes four parameters

previousValue （第一项的值或者上一次叠加的结果值，或者是提供的初始值（initialValue））
currentValue （数组中当前被处理的元素）
index （当前元素在数组中的索引）
array （数组本身）
initialValue （作为第一次调用 callback 的第一个参数，可以控制返回值的格式）不传默认为0

例子1：数组求和

    const arr = [1, 2, 3, 4, 5]
    let sum = arr.reduce(function (pre, cur) {
        console.log(pre, cur) //输出的是第一项的值或上一次叠加的结果，正在被处理的元素
        return pre + cur
    })
    console.log(sum) //15

例子2：不要使用JavaScript内置的parseInt()函数，利用map和reduce操作实现一个string2int()函数

思路：1.先把字符串13579先变成Array——[1, 3, 5, 7, 9]
2.再利用reduce()就可以写出一个把字符串转换为Number的函数。

    const s = '13579'
    function string2int(s) {
        let arr = s.split('').map(function(x){
            return +x;
        })
        console.log(arr) //[1, 3, 5, 7, 9]
        let arr_reduce = arr.reduce(function(prev,res){
            console.log(prev, res)
            return prev*10+res;
        })
        console.log(arr_reduce) //13579
        return arr_reduce

    }
    string2int(s)

1.split() 方法用于把一个字符串分割成字符串数组。
格式：

stringObject.split(separator,howmany)

separator 必需。字符串或正则表达式，从该参数指定的地方分割 stringObject。如果是空字符串”，按每个单词分割
howmany 可选。该参数可指定返回的数组的最大长度。如果设置了该参数，返回的子串不会多于这个参数指定的数组。如果没有设置该参数，整个字符串都会被分割，不考虑它的长度。
2.parsenInt和+

js提供了parseInt()和parseFloat()两个转换函数。前者把值转换成整数，后者把值转换成浮点数。

在字符串前面输入‘+’也能将字符转化成数值，输出如下：

例子3：求一串字符串中每个字母出现的次数

暂时不会

参考：

https://www.cnblogs.com/jianxian/p/10582683.html

https://blog.csdn.net/baidu_36065997/article/details/79079880

JS higher-order functions -------- map, reduce, filter

A, filter

1. Grammar

Two, map

Three, reduce

例子1：数组求和

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