search
HomeWeb Front-endJS TutorialSeveral methods of merging arrays in JS and comparison of advantages and disadvantages_Basic knowledge

This article belongs to the basic skills of JavaScript. We will learn various common methods of combining/merging two JS arrays and compare the advantages and disadvantages of various methods.

Let’s take a look at the specific scenario first:

Copy code The code is as follows:

var q = [5, 5, 1, 9, 9, 6, 4, 5, 8];
var b = [ "tie", "mao", "csdn", "ren", "fu", "fei" ];

Obviously, the result of simple concatenation of arrays q and b is:

Copy code The code is as follows:

[
5, 5, 1, 9, 9, 6, 4, 5, 8,
"tie", "mao", "csdn", "ren", "fu", "fei"
]

concat(..) method

The most common usage is as follows:

Copy code The code is as follows:

var c = q.concat( b );

q; // [5,5,1,9,9,6,4,5,8]
b; // ["tie","mao","csdn","ren","fu","fei"];

c; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

As you can see, c is a brand new array, representing the combination of the two arrays q and b, but q and b are useless now, right?

If the q array has 10,000 elements, and the b array also has 10,000 elements? Then the array c now has 20,000 elements, and this method takes up twice the memory.

"That's no problem!", you may think. Just leave q and b empty, and then they will be garbage collected, right? Problem solved!

Copy code The code is as follows:

q = b = null; // `q` and `b` are now garbage collected

Um? If the arrays are small, then there is no problem. But for large arrays, or when repeated processing is required multiple times, the memory is limited, and it also needs to be optimized.

Loop insertion

OK, let’s try adding the contents of one array to another, using the Array#push() method:

Copy code The code is as follows:

// Insert array `b` into `q`
for (var i=0; i q.push( b[i] );
}

q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

b = null;

Now, q stores the contents of two original arrays (q b).

It seems that the memory optimization has been done well.

But what if the q array is small and b is large? For the sake of memory and speed, you want to insert the smaller q in front of b. No problem, just use the unshift() method instead of push() That’s it, and the corresponding cycle needs to be traversed from large to small:

Copy code The code is as follows:

// `q` into `b`:
for (var i=q.length-1; i >= 0; i--) {
b.unshift( q[i] );
}

b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

q = null;

Practical Tips

Sadly, for loops are boring and difficult to maintain. Can we do better?
Let’s try Array#reduce first:

Copy code The code is as follows:

// `b` onto `q`:
q = b.reduce( function(coll,item){
coll.push(item);
Return coll;
}, q );

q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

// or `q` into `b`:
b = q.reduceRight( function(coll,item){
coll.unshift(item);
Return coll;
}, b );

b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

Array#reduce() and Array#reduceRight() are very fancy, but a bit cumbersome, and most people can’t remember them. The => arrow-functions in JS specification 6 can greatly reduce the amount of code. , but it requires a function call for each array element, which is also a very poor method.
So what about the code below?

Copy code The code is as follows:

// `b` onto `q`:
q.push.apply( q, b );

q; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

// or `q` into `b`:
b.unshift.apply( b, q );

b; // [5,5,1,9,9,6,4,5,8,"tie","mao","csdn","ren","fu","fei"]

BIG is higher, right!? Especially the unshift() method does not need to consider the reverse order as before. The ES6 spread operator (spread operator, plus... prefix) is even higher: a. push( ...b ) or b.unshift( ...a )

However, in fact this method is still too optimistic. In both cases, whether a or b is passed to apply() as the second parameter (the first parameter is internal when calling Function in apply mode Become this (i.e. context, context, scope), or use... the expansion operator. In fact, the array will be broken up into arguments of the function.
The first major problem is that it takes up double the memory (temporarily, of course!) because the array needs to be copied to the function stack. In addition, different JS engines have different implementation algorithms, which may limit The number of parameters that can be passed to the function.

If one million elements are added to the array, it will definitely exceed the size allowed by the function stack, regardless of push() or unshift() call. This method is only available when there are thousands of elements, so it must be limited It cannot exceed a certain range.

Note: You can also try splice(), and you will definitely find that it has the same restrictions as push(..)/unshift(..).

One option is to continue using this method, but in batches:

Copy code The code is as follows:

function combineInto(q,b) {
var len = q.length;
for (var i=0; i               // Process 5000 items at a time
b.unshift.apply( b, q.slice( i, i 5000 ) );
}
}

Wait, we’re hurting the readability of our code (and even performance!). Let’s end this journey before we give up.

Summary

Array#concat() is the tried and tested method for combining two (or more) arrays. But it creates a new array rather than modifying an existing one.

There are many alternative methods, but they all have different advantages and disadvantages, and you need to choose according to the actual situation.

There are various advantages/disadvantages listed above, perhaps the best (including those not listed) are reduce(..) and reduceRight(..)

Whatever you choose, think critically about your array merging strategy and not take it for granted.

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
如何使用JS和百度地图实现地图平移功能如何使用JS和百度地图实现地图平移功能Nov 21, 2023 am 10:00 AM

如何使用JS和百度地图实现地图平移功能百度地图是一款广泛使用的地图服务平台,在Web开发中经常用于展示地理信息、定位等功能。本文将介绍如何使用JS和百度地图API实现地图平移功能,并提供具体的代码示例。一、准备工作使用百度地图API前,首先需要在百度地图开放平台(http://lbsyun.baidu.com/)上申请一个开发者账号,并创建一个应用。创建完成

js字符串转数组js字符串转数组Aug 03, 2023 pm 01:34 PM

js字符串转数组的方法:1、使用“split()”方法,可以根据指定的分隔符将字符串分割成数组元素;2、使用“Array.from()”方法,可以将可迭代对象或类数组对象转换成真正的数组;3、使用for循环遍历,将每个字符依次添加到数组中;4、使用“Array.split()”方法,通过调用“Array.prototype.forEach()”将一个字符串拆分成数组的快捷方式。

如何使用JS和百度地图实现地图热力图功能如何使用JS和百度地图实现地图热力图功能Nov 21, 2023 am 09:33 AM

如何使用JS和百度地图实现地图热力图功能简介:随着互联网和移动设备的迅速发展,地图成为了一种普遍的应用场景。而热力图作为一种可视化的展示方式,能够帮助我们更直观地了解数据的分布情况。本文将介绍如何使用JS和百度地图API来实现地图热力图的功能,并提供具体的代码示例。准备工作:在开始之前,你需要准备以下事项:一个百度开发者账号,并创建一个应用,获取到相应的AP

如何使用JS和百度地图实现地图多边形绘制功能如何使用JS和百度地图实现地图多边形绘制功能Nov 21, 2023 am 10:53 AM

如何使用JS和百度地图实现地图多边形绘制功能在现代网页开发中,地图应用已经成为常见的功能之一。而地图上绘制多边形,可以帮助我们将特定区域进行标记,方便用户进行查看和分析。本文将介绍如何使用JS和百度地图API实现地图多边形绘制功能,并提供具体的代码示例。首先,我们需要引入百度地图API。可以利用以下代码在HTML文件中导入百度地图API的JavaScript

js中new操作符做了哪些事情js中new操作符做了哪些事情Nov 13, 2023 pm 04:05 PM

js中new操作符做了:1、创建一个空对象,这个新对象将成为函数的实例;2、将新对象的原型链接到构造函数的原型对象,这样新对象就可以访问构造函数原型对象中定义的属性和方法;3、将构造函数的作用域赋给新对象,这样新对象就可以通过this关键字来引用构造函数中的属性和方法;4、执行构造函数中的代码,构造函数中的代码将用于初始化新对象的属性和方法;5、如果构造函数中没有返回等等。

用JavaScript模拟实现打字小游戏!用JavaScript模拟实现打字小游戏!Aug 07, 2022 am 10:34 AM

这篇文章主要为大家详细介绍了js实现打字小游戏,文中示例代码介绍的非常详细,具有一定的参考价值,感兴趣的小伙伴们可以参考一下。

php可以读js内部的数组吗php可以读js内部的数组吗Jul 12, 2023 pm 03:41 PM

php在特定情况下可以读js内部的数组。其方法是:1、在JavaScript中,创建一个包含需要传递给PHP的数组的变量;2、使用Ajax技术将该数组发送给PHP脚本。可以使用原生的JavaScript代码或者使用基于Ajax的JavaScript库如jQuery等;3、在PHP脚本中,接收传递过来的数组数据,并进行相应的处理即可。

js是什么编程语言?js是什么编程语言?May 05, 2019 am 10:22 AM

js全称JavaScript,是一种具有函数优先的轻量级,直译式、解释型或即时编译型的高级编程语言,是一种属于网络的高级脚本语言;JavaScript基于原型编程、多范式的动态脚本语言,并且支持面向对象、命令式和声明式,如函数式编程。

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
2 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
R.E.P.O. Best Graphic Settings
2 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
R.E.P.O. How to Fix Audio if You Can't Hear Anyone
3 weeks agoBy尊渡假赌尊渡假赌尊渡假赌

Hot Tools

SAP NetWeaver Server Adapter for Eclipse

SAP NetWeaver Server Adapter for Eclipse

Integrate Eclipse with SAP NetWeaver application server.

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

MantisBT

MantisBT

Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

SublimeText3 Linux new version

SublimeText3 Linux new version

SublimeText3 Linux latest version

PhpStorm Mac version

PhpStorm Mac version

The latest (2018.2.1) professional PHP integrated development tool