Several iteration methods in JavaScript

Iteration

Definition :
1. Iterate refers to visiting each item in the list one by one in a certain order. For example, the for statement.
2. Loop refers to the repeated execution of the same piece of code when the conditions are met. For example, the while statement.
3. Traversal refers to visiting each node in the tree structure according to certain rules, and each node is only visited once.
4. Recursion refers to the behavior of a function continuously calling itself. For example, programmatically output the famous Fibonacci sequence.

Iterative methods :
1, Every
used to query the array for each if both meet certain conditions

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.every(function(item,index){
    
    
      return (item > 5);   // 判断传入的值是否全部都大于5
})
console.log(number);   // false 只要有一个不满足就为 false

2.
Each item in the some query array satisfies certain conditions

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.every(function(item,index){
    
    
      return (item > 5);   // 判断传入的值哪些项大于5
})
console.log(number);   // false 只要有一个满足就为true

3. Filter
filters the items that meet the conditions to form a new array

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.filter(function(item,index){
    
    
      return (item > 5);   // 判断传入的值哪些项大于5
})
console.log(number);   // [6, 7, 8, 9]

4. The map
forms a new array by calculating the items in the original array

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.map(function(item,index){
    
    
      return item - 1;   // 判断传入的值哪些项大于5
})
console.log(number);   // [0, 1, 2, 3, 4, 5, 6, 7, 8]

5. ForEach
passes each item of the array

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.forEach(function(item,index){
    
    
      console.log(item);   // [1, 2, 3, 4, 5, 6, 7, 8, 9]
})

6.
Calculate the cumulative value of the preceding and following items of the reduce array

var num = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var number = num.reduce(function(prev,cur,index){
    
    
      return prev + cur;
})
console.log(number);   // 1+2+3...+9 = 45 本质上就是数组中的项 累计运算的过程

note

The forEach iteration method is essentially the same as the for loop, but when using this method, three parameters are passed in, and the first item is passed in when outputting, that is, each item in the output array. If you pass in For the second item, the index value of the array will be iterated in a loop. If the third item is passed in, it will iterate the entire num array.

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Origin blog.csdn.net/weixin_43176019/article/details/109101960