JavaScript's map loop, foreach Loop, filter loop

1. Map loop

Let Arr=[1,2,3,4];arr.map (function (Value,key,arr) {//value, index, array (default = selected array)    return item; Returns an array of all undefined if no return, number is index +1 (key+1)})//returns an array

2. Foreach Loop

Let Arr=[1,2,3,4];arr.map (function (Value,key,arr) {//value, index, array (default = selected array)    return item; Do anything})//return only undefined

3. Filter loop

Let Arr=[1,2,3,4];arr.map (function (Value,key,arr) {//value, index, array (default = selected array)    return false; Based on true and false to see if the original array is returned})//false return [],true returns only the original array

4. Reduce cycle (from beginning to end)

Let Arr=[1,2,3,4];arr.map (function (Fristvalue,nextvalue,key,arr) {//Array first value (first occurrence), next value of array, index, array (default is selected array)    return fristvalue+nextvalue; Add all values, always iterate})//Return the result of the return iteration

5. Reduceright cycle (from tail to head)

Let Arr=[1,2,3,4];arr.map (function (Fristvalue,nextvalue,key,arr) {//array last value (first occurrence), array next value, index, array (default = selected array)    return fristvalue+nextvalue; Add all values, always iterate})//Return the result of the return iteration

　　

JavaScript's map loop, foreach Loop, filter loop