Commonly used traversal methods for javascript arrays (code examples)

What this article brings to you is about the commonly used traversal methods (code examples) of JavaScript arrays. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Preface

This article mainly introduces common array traversal methods: forEach, map, filter, find, every, some, reduce, they all have one thing in common: they will not change the original array.

1. forEach: Traverse the array

var colors = ["red","blue","green"];
// ES5遍历数组方法
for(var i = 0; i <pre class="brush:php;toolbar:false">// ES6 forEach
colors.forEach(function(color){
 console.log(color);//red blue green
});

Let’s look at an example again: traverse the values ​​in the array and calculate the sum

var numbers = [1,2,3,4,5];
var sum = 0;
numbers.forEach(number=>sum+=number)
console.log(sum)//15

2. Map: Map an array into another array

map processes each element of the array through the specified function and returns the new array after processing. Map does not change the original array.

The difference between forEach and map is that forEach has no return value.
map needs to return a value. If no return is given, it will return undefined by default

Usage scenario 1
Assume that there is a numerical array (A), and the values ​​in the A array are doubled in the form B array

var numbers = [1,2,3];
var doubledNumbers = [];
// es5写法
for(var i = 0; i <pre class="brush:php;toolbar:false">// es6 map方法
var doubled = numbers.map(function(number){
   return number * 2;
})
console.log(doubled);//[2,4,6]

Usage scenario 2 Assume there is an object array (A), and store the value of a certain attribute of the object in the A array into the B array

var cars = [
  {model:"Buick",price:"CHEAP"},
  {model:"BMW",price:"expensive"}
];
var prices = cars.map(function(car){
    return car.price;
})
console.log(prices);//["CHEAP", "expensive"]

3. Filter: Find all elements that meet the specified conditions from the array

filter() detects numeric elements and returns an array of all elements that meet the conditions. filter() does not change the original array.

Usage scenario 1: Assume there is an object array (A), get the specified type of object in the array and put it into the B array

var porducts = [
  {name:"cucumber",type:"vegetable"},
  {name:"banana",type:"fruit"},
  {name:"celery",type:"vegetable"},
  {name:"orange",type:"fruit"}
];
// es5写法
var filteredProducts = [];
for(var i = 0; i <pre class="brush:php;toolbar:false">// es6 filter
var filtered2 = porducts.filter(function(product){
  return product.type === "vegetable";
})
console.log(filtered2);

Usage scenario 2: Assume there is An object array (A), filter out objects that do not meet the following conditions
Conditions: The quantity of vegetables is greater than 0, and the price is less than 10

var products = [
  {name:"cucumber",type:"vegetable",quantity:0,price:1},
  {name:"banana",type:"fruit",quantity:10,price:16},
  {name:"celery",type:"vegetable",quantity:30,price:8},
  {name:"orange",type:"fruit",quantity:3,price:6}
];
products = products.filter(function(product){
    return product.type === "vegetable" 
    && product.quantity > 0 
    && product.price <p><strong>Usage scenario 3</strong>: Assume there are two arrays (A,B), based on the id value in A, filter out the data that does not match the B array</p><pre class="brush:php;toolbar:false">var post = {id:4,title:"Javascript"};
var comments = [
   {postId:4,content:"Angular4"},
   {postId:2,content:"Vue.js"},
   {postId:3,content:"Node.js"},
   {postId:4,content:"React.js"},
];
function commentsForPost(post,comments){
   return comments.filter(function(comment){
     return comment.postId === post.id;
   })
}
console.log(commentsForPost(post,comments));//[{postId:4,content:"Angular4"},{postId:4,content:"React.js"}]

4. find: Returns the first element of the array that passes the test (judged within the function) Value

Its parameter is a callback function, and all array members execute the callback function in sequence until the first member whose return value is true is found, and then returns that member. If there are no matching members, undefined is returned.
Usage scenario 1
Assume there is an object array (A), find the object that meets the conditions

 var users = [
  {name:"Jill"},
  {name:"Alex",id:2},
  {name:"Bill"},
  {name:"Alex"}
 ];
// es5方法
 var user;
 for(var i = 0; i <pre class="brush:php;toolbar:false">// es6 find
user = users.find(function(user){
  return user.name === "Alex";
})
console.log(user);// {name:"Alex",id:2}找到后就终止循环

Usage scenario 2: Assume there is an object array (A), according to Specify the conditions of the object to find the objects in the array that meet the conditions

var posts = [
 {id:3,title:"Node.js"},
 {id:1,title:"React.js"}
];
var comment = {postId:1,content:"Hello World!"};
function postForComment(posts,comment){
 return posts.find(function(post){
   return post.id === comment.postId;
 })
}
console.log(postForComment(posts,comment));//{id: 1, title: "React.js"}

5. every&some

every: Whether each element in the array meets the specified conditions

some: Whether there are elements in the array that meet the specified conditions

Usage scenario 1: Calculate whether each computer operating system in the object array is available. If the operating system is greater than 16 bits, it means it is available, otherwise it is not available

//ES5方法
var computers = [
 {name:"Apple",ram:16},
 {name:"IBM",ram:4},
 {name:"Acer",ram:32}
];
var everyComputersCanRunProgram = true;
var someComputersCanRunProgram = false;
for(var i = 0; i <pre class="brush:php;toolbar:false">//ES6 some every 
var every = computers.every(function(computer){
  return computer.ram > 16;
})
console.log(every);//false
var some = computers.some(function(computer){
 return computer.ram > 16;
})
console.log(some);//true

In a nutshell: Every: One true is true; Some: One false is false

##Usage scenario 2: Assume there is a registration page, Determine whether the length of all input content is greater than 0

function Field(value){
  this.value = value;
}
Field.prototype.validate = function(){
  return this.value.length > 0;
}
//ES5方法
var username = new Field("henrywu");
var telephone = new Field("18888888888");
var password = new Field("my_password");
console.log(username.validate());//true
console.log(telephone.validate());//true
console.log(password.validate());//true
//ES6 some every
var fields = [username,telephone,password];
var formIsValid = fields.every(function(field){
 return field.validate();
})
console.log(formIsValid);//true
if(formIsValid){
 // 注册成功
}else{
  // 给用户一个友善的错误提醒
}

6. reduce: combine the array into a value

reduce() method receives a method as an accumulator, in the array Each value of (from left to right) starts to be merged into one value.

Usage scenario 1: Calculate the sum of all values ​​in the array

 var numbers = [10,20,30];
 var sum = 0;
//es5 方法
for(var i = 0; i <pre class="brush:php;toolbar:false">// es6 reduce
var sumValue = numbers.reduce(function(sum2,number2){
  console.log(sum2);//0 10 30 60
  return sum2 + number2;
},0);//sum2初始值为0
console.log(sumValue);

Usage scenario 2: Extract an attribute of the object in the array to another In the array

 var primaryColors = [
   {color:"red"},
   {color:"yellow"},
   {color:"blue"}
 ];
 var colors = primaryColors.reduce(function(previous,primaryColor){
    previous.push(primaryColor.color);
    return previous;
 },[]);
 console.log(colors);//["red", "yellow", "blue"]

Usage scenario 3: Determine whether the brackets in the string are symmetrical

function balancedParens(string){
  return !string.split("").reduce(function(previous,char){
    if(previous <p class="comments-box-content"></p>

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