Compilation of JavaScript error-prone knowledge points-JS Tutorial-php.cn

Home

Web Front-end

JS Tutorial

Compilation of JavaScript error-prone knowledge points

黄舟

Feb 07, 2017 pm 02:35 PM

javascript

Preface

This article is about some error-prone knowledge points that I collected and organized in the process of learning JavaScript. It will focus on variable scope, type comparison, this pointer, function parameters, closure issues and object copying and The six aspects of assignment are introduced and explained from the shallower to the deeper, which also involves some ES6 knowledge points.

JavaScript knowledge points

1. Variable scope

var a = 1;

function test() {
    var a = 2;

    console.log(a); // 2
}

test();

Copy after login

A is declared and assigned in the function scope above, and it is above the console, so the output follows the proximity principle a is equal to 2.

var a = 1;

function test2() {
    console.log(a); // undefined

    var a = 2;
}

test2();

Copy after login

Although a is declared and assigned in the function scope above, it is located under the console, and the a variable is promoted. It has been declared but has not been assigned a value during output, so "undefined" is output.

var a = 1;

function test3() {
    console.log(a); // 1

    a = 2;
}

test3();

Copy after login

A in the function scope above is reassigned, not re-declared, and is located under the console, so a in the global scope is output.

let b = 1;

function test4() {
    console.log(b); // b is not defined

    let b = 2;
}

test4();

Copy after login

The ES6 let is used in the function scope above to redeclare the variable b. Unlike var, let does not have the function of variable promotion, so the output error "b is not defined" is reported.

function test5() {
    let a = 1;

    {
        let a = 2;
    }
    
    console.log(a); // 1
}

test5();

Copy after login

In the function scope above, let is used to declare a as 1, and a is declared as 2 in the block-level scope. Because the console is not in the block-level scope within the function, 1 is output. .

2. Type comparison

var arr = [],
    arr2 = [1];

console.log(arr === arr2); // false

Copy after login

Comparison of two different arrays above, console is false.

var arr = [],
    arr2 = [];

console.log(arr === arr2); // false

Copy after login

Comparison of the two identical arrays above, because the comparison between arrays and arrays is always false, so the console is false.

var arr = [],
    arr2 = {};

console.log(typeof(arr) === typeof(arr2)); // true

Copy after login

The above uses typeof to compare arrays and objects. Because typeof obtains NULL, the types of arrays and objects are all object, so the console is true.

var arr = [];

console.log(arr instanceof Object); // true
console.log(arr instanceof Array); // true

Copy after login

The above uses instanceof to determine whether a variable belongs to an instance of an object. Because arrays are also a type of object in JavaScript, both consoles are true.

3.this points to

var obj = {
    name: &#39;xiaoming&#39;,
    getName: function () {
        return this.name;
    }
};

console.log(obj.getName());  // &#39;xiaoming&#39;

Copy after login

This in the object method above points to the object itself, so "xiaoming" is output.

var obj = {
    myName: &#39;xiaoming&#39;,
    getName: function () {
        return this.myName;
    }
};

var nameFn = obj.getName;console.log(nameFn()); // undefined

Copy after login

The method in the object is assigned to a variable above. At this time, this in the method will no longer point to the obj object, but to the window object, so the console is "undefined".

var obj = {
    myName: &#39;xiaoming&#39;,
    getName: function () {
        return this.myName;
    }
};

var obj2 = {
    myName: &#39;xiaohua&#39;
};
    
var nameFn = obj.getName;console.log(nameFn.apply(obj2)); // &#39;xiaohua&#39;

Copy after login

The above also assigns the method in the obj object to the variable nameFn, but points this to the obj2 object through the apply method, so the final console is 'xiaohua'.

4. Function parameters

function test6() {
    console.log(arguments); // [1, 2]
}

test6(1, 2);

Copy after login

The above uses the arguments object in the function to obtain the parameter array passed into the function, so the output array is [1, 2].

function test7 () {
    return function () {
        console.log(arguments); // 未执行到此，无输出
    }
}

test7(1, 2);

Copy after login

The above also uses arguments to obtain parameters, but because test7(1, 2) does not execute the function in return, there is no output. If test7(1, 2)(3, 4) is executed, it will output [ 3, 4].

var args = [1, 2];

function test9() {
    console.log(arguments); // [1, 2, 3, 4]
}

Array.prototype.push.call(args, 3, 4);

test9(...args);

Copy after login

上方利用Array.prototype.push.call()方法向args数组中插入了3和4，并利用ES6延展操作符(…)将数组展开并传入test9，所以console为[1, 2, 3, 4]。

5.闭包问题

var elem = document.getElementsByTagName(&#39;div&#39;); // 如果页面上有5个div

for(var i = 0; i < elem.length; i++) {
    elem[i].onclick = function () {
        alert(i); // 总是5
    };
}

Copy after login

上方是一个很常见闭包问题，点击任何div弹出的值总是5，因为当你触发点击事件的时候i的值早已是5，可以用下面方式解决：

var elem = document.getElementsByTagName(&#39;div&#39;); // 如果页面上有5个div

for(var i = 0; i < elem.length; i++) {
    (function (w) {
        elem[w].onclick = function () {
            alert(w); // 依次为0,1,2,3,4
        };
    })(i);
}

Copy after login

在绑定点击事件外部封装一个立即执行函数，并将i传入该函数即可。

6.对象拷贝与赋值

var obj = {
    name: &#39;xiaoming&#39;,
    age: 23
};
    
var newObj = obj;

newObj.name = &#39;xiaohua&#39;;

console.log(obj.name); // &#39;xiaohua&#39;
console.log(newObj.name); // &#39;xiaohua&#39;

Copy after login

上方我们将obj对象赋值给了newObj对象，从而改变newObj的name属性，但是obj对象的name属性也被篡改，这是因为实际上newObj对象获得的只是一个内存地址，而不是真正的拷贝，所以obj对象被篡改。

var obj2 = {
    name: &#39;xiaoming&#39;,
    age: 23
};
    
var newObj2 = Object.assign({}, obj2, {color: &#39;blue&#39;});

newObj2.name = &#39;xiaohua&#39;;console.log(obj2.name); // &#39;xiaoming&#39;

console.log(newObj2.name); // &#39;xiaohua&#39;
console.log(newObj2.color); // &#39;blue&#39;

Copy after login

上方利用Object.assign()方法进行对象的深拷贝可以避免源对象被篡改的可能。因为Object.assign() 方法可以把任意多个的源对象自身的可枚举属性拷贝给目标对象，然后返回目标对象。

var obj3 = {
    name: &#39;xiaoming&#39;,
    age: 23
};
    
var newObj3 = Object.create(obj3);

newObj3.name = &#39;xiaohua&#39;;console.log(obj3.name); // &#39;xiaoming&#39;

console.log(newObj3.name); // &#39;xiaohua&#39;

Copy after login

我们也可以使用Object.create()方法进行对象的拷贝，Object.create()方法可以创建一个具有指定原型对象和属性的新对象。

结语

学习JavaScript是一个漫长的过程，不能一蹴而就。希望本文介绍的几点内容能够帮助学习JavaScript的同学更加深入的了解和掌握JavaScript的语法，少走弯路。

以上就是JavaScript易错知识点整理的内容，更多相关内容请关注PHP中文网（www.php.cn）！

Statement of this Website

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

Hot AI Tools

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress images for free

Clothoff.io

AI clothes remover

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Assassin's Creed Shadows: Seashell Riddle Solution

4 weeks ago By DDD

What's New in Windows 11 KB5054979 & How to Fix Update Issues

3 weeks ago By DDD

Where to find the Crane Control Keycard in Atomfall

4 weeks ago By DDD

Roblox: Dead Rails - How To Complete Every Challenge

1 months ago By DDD

How to fix KB5055523 fails to install in Windows 11?

2 weeks ago By DDD

Hot Tools

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

God-level code editing software (SublimeText3)

Hot Topics

Where is the login entrance for gmail email?

7742

Java Tutorial

1643

CakePHP Tutorial

1397

Laravel Tutorial

1291

PHP Tutorial

1233

Related knowledge

How to implement an online speech recognition system using WebSocket and JavaScript Dec 17, 2023 pm 02:54 PM

How to use WebSocket and JavaScript to implement an online speech recognition system Introduction: With the continuous development of technology, speech recognition technology has become an important part of the field of artificial intelligence. The online speech recognition system based on WebSocket and JavaScript has the characteristics of low latency, real-time and cross-platform, and has become a widely used solution. This article will introduce how to use WebSocket and JavaScript to implement an online speech recognition system.

WebSocket and JavaScript: key technologies for implementing real-time monitoring systems Dec 17, 2023 pm 05:30 PM

WebSocket and JavaScript: Key technologies for realizing real-time monitoring systems Introduction: With the rapid development of Internet technology, real-time monitoring systems have been widely used in various fields. One of the key technologies to achieve real-time monitoring is the combination of WebSocket and JavaScript. This article will introduce the application of WebSocket and JavaScript in real-time monitoring systems, give code examples, and explain their implementation principles in detail. 1. WebSocket technology

How to use JavaScript and WebSocket to implement a real-time online ordering system Dec 17, 2023 pm 12:09 PM

Introduction to how to use JavaScript and WebSocket to implement a real-time online ordering system: With the popularity of the Internet and the advancement of technology, more and more restaurants have begun to provide online ordering services. In order to implement a real-time online ordering system, we can use JavaScript and WebSocket technology. WebSocket is a full-duplex communication protocol based on the TCP protocol, which can realize real-time two-way communication between the client and the server. In the real-time online ordering system, when the user selects dishes and places an order

How to implement an online reservation system using WebSocket and JavaScript Dec 17, 2023 am 09:39 AM

How to use WebSocket and JavaScript to implement an online reservation system. In today's digital era, more and more businesses and services need to provide online reservation functions. It is crucial to implement an efficient and real-time online reservation system. This article will introduce how to use WebSocket and JavaScript to implement an online reservation system, and provide specific code examples. 1. What is WebSocket? WebSocket is a full-duplex method on a single TCP connection.

JavaScript and WebSocket: Building an efficient real-time weather forecasting system Dec 17, 2023 pm 05:13 PM

JavaScript and WebSocket: Building an efficient real-time weather forecast system Introduction: Today, the accuracy of weather forecasts is of great significance to daily life and decision-making. As technology develops, we can provide more accurate and reliable weather forecasts by obtaining weather data in real time. In this article, we will learn how to use JavaScript and WebSocket technology to build an efficient real-time weather forecast system. This article will demonstrate the implementation process through specific code examples. We

Simple JavaScript Tutorial: How to Get HTTP Status Code Jan 05, 2024 pm 06:08 PM

JavaScript tutorial: How to get HTTP status code, specific code examples are required. Preface: In web development, data interaction with the server is often involved. When communicating with the server, we often need to obtain the returned HTTP status code to determine whether the operation is successful, and perform corresponding processing based on different status codes. This article will teach you how to use JavaScript to obtain HTTP status codes and provide some practical code examples. Using XMLHttpRequest

How to use insertBefore in javascript Nov 24, 2023 am 11:56 AM

Usage: In JavaScript, the insertBefore() method is used to insert a new node in the DOM tree. This method requires two parameters: the new node to be inserted and the reference node (that is, the node where the new node will be inserted).

JavaScript and WebSocket: Building an efficient real-time image processing system Dec 17, 2023 am 08:41 AM

JavaScript is a programming language widely used in web development, while WebSocket is a network protocol used for real-time communication. Combining the powerful functions of the two, we can create an efficient real-time image processing system. This article will introduce how to implement this system using JavaScript and WebSocket, and provide specific code examples. First, we need to clarify the requirements and goals of the real-time image processing system. Suppose we have a camera device that can collect real-time image data

See all articles