What are the iteration methods in JavaScript?

Iteration methods in JavaScript: 1. every(), which can query whether each element in the array satisfies a certain condition; 2. some(), which can query whether each element in the array satisfies certain conditions. Condition; 3. filter(); 4. map(); 5. forEach(); 6. reduce().

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Definition:

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

Iteration method in JavaScript:

1. every()

is used to query in the array Whether each satisfies a certain condition

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, some()

Query each item in the array to satisfy 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()

Filter the items that meet the conditions and 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. map()

Compose 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()

Every item passed in the array Item

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, reduce()

The previous item and the last item of the array are used to calculate the cumulative value

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. 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 subscript value of the index of the array will be iterated in a loop. If the third item is passed in, the entire num array will be iterated in a loop.

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javascript advanced tutorial]

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