Array traversal method in JavaScript

1. forEach

grammar:array.forEach(function(item, index, origin){})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to each item of the array
3. Original array
   Function: Traversing the array
   Return value: The return value is undefined If you write return, the return value is also undefined

var arr = [1,2,3,4,5]
var num = arr.forEach(function(item,index,origin){
    
    
          return item})
console.log(num)  //---> 原数组

2. map

grammar:array.map(function (item, index, origin) {})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to the actual value of each item in the array
3. Original array
   Function: Mapping array
   Return value: Return an array with the same length as the original array, and each value of the returned array depends on how to write the return of the parameter

var arr = [1,2,3,4,5]
var num = arr.map(function(item,index,origin){
    
    
          return item*2})
console.log(num)  //---> [2,4,6,8,10]

3. filter

grammar:array.filter(function (item, index, origin) {})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to the actual value of each item in the array
3. Original array
   Function: Filter the array
   Return value: The new array after filtering the array, the filter condition depends on how to write the return of the parameter

var arr = [1,2,3,4,5]
var num = arr.filter(function(item,index,origin){
    
    
          return item > 2})
console.log(num)  //---> [3,4,5]

4. find

grammar:array.find(function (item, index, origin) {})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to the actual value of each item in the array
3. Original array
   Function: Find data
   Return value: The first data found in the array (not an array)

var arr = [1,2,3,4,5]
var num = arr.find(function(item,index,origin){
    
    
          return item > 3})
console.log(num)  //---> 4

5. findIndex

grammar:array.findIndex(function (item, index, origin) {})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to the actual value of each item in the array
3. Original array
   Function: Find the subscript of the first occurrence of the data
   Return value: The subscript of the first occurrence of the data found in the array

var arr = [1,2,3,4,5]
var num = arr.findIndex(function(item,index,origin){
    
    
          return item > 3})
console.log(num)  //---> 3

6. every

grammar:array.every(function(item, index, origin){})
parameter:

1. The actual value of the first item in the array
2. The subscript corresponding to the actual value of the first item in the array
3. Original array
   Function: Determine whether all the data in the array meet the conditions
   Return value: a Boolean value
   true —> all the data in the array meet the conditions
   false —> at least one data in the array does not meet the conditions

var arr = [1,2,3,4,5]
var num = arr.every(function(item,index,origin){
    
    
          return item > 0})
console.log(num)  //---> true

7. some

grammar:array.some(function (item, index, origin) {})
parameter:

1. The actual value of each item in the array
2. The subscript corresponding to the actual value of each item in the array
3. Original array
   Function: Determine whether there is an item in the array that satisfies the condition
   Return value: A Boolean value
   true: indicates that at least one item in the array meets the condition
   false: none of the data in the array meets the condition

var arr = [1,2,3,4,5]
var num = arr.some(function(item,index,origin){
    
    
          return item > 5})
console.log(num)  //---> false

8. reduce

grammar:array.reduce(function (prve, item, index, origin) {}, init)
The parameters of the reduce method:

1. If the second parameter init is passed, the number of executions is the same as the length of the array
2. If the second parameter init is not passed, the default first value is the value of the first item of the array, and the number of executions is reduced by 1 from the length of the array. The
   meaning of the 4 formal parameters in the function of parameter 1:
3. Represents the initial value or the value of the first item of the array (depending on whether init is passed)
4. The actual value of each item in the array
5. The subscript corresponding to the actual value of each item in the array
6. Original array
   Function: Accumulation (superposition)
   Return value: The value obtained after the loop runs

do not pass the second parameter

var arr = [1,2,3,4,5]
var num = arr.reduce(function(prve,item,index,origin){
    
    
          return prve + item})
console.log(num)  //---> 15

Pass the second parameter

var arr = [1,2,3,4,5]
var num = arr.reduce(function(prve,item,index,origin){
    
    
          return prve * item},2)
console.log(num)  //---> 240