Web Front-end
JS Tutorial
Detailed explanation of examples of constructor and new operator in JavaScript
Detailed explanation of examples of constructor and new operator in JavaScript
Detailed explanation of examples of constructors and new operators in JavaScript
｛…｝ syntax allows the creation of one object, but if we need to create multiple similar objects, we Requires use of constructor and "new" operator.
Constructor
Constructor is technically a normal function, but there are generally two conventions:
1. The first letter of their name is capitalized.
2. They should only be executed using the new operator.
Example:
function User(name) {
this.name = name;
this.isAdmin = false;
}
let user = new User("Jack");
alert(user.name); // Jack
alert(user.isAdmin); // falseLet’s take a look at what is done behind the scenes when executing new User()?
1. First create an empty object and then assign it to this.
2. Execute the function, usually modify this object and add some new attributes.
3. this is returned.
In other words, the content of new User() is as follows:
function User(name) {
// this = {}; (implicitly)
// we add properties to this
this.name = name;
this.isAdmin = false;
// return this; (implicitly)
}So the result of new User("Jack") is The following code is consistent:
let user = {
name: "Jack",
isAdmin: false
}; Now we want to create other objects, we can use new User("Ann"), new User("Alice"), etc. Shorter and easier to read than using text every time to describe an object.
The main purpose of the constructor at this time is to achieve code reuse for creating objects.
Let us pay attention again:
Technically, any function can be used as a constructor, i.e. any function can be called using new and also perform the algorithm described above.
Capitalizing the first letter is just a general convention to make it clearer that it is a Korean constructor and should be called using new.
new function(){…}
If we have multiple lines of code for creating a complex object, we can wrap them using a constructor, the code is as follows:
let user = new function() {
this.name = “John”;
this.isAdmin = false;
// ...other code for user creation
// maybe complex logic and statements
// local variables etc
};The constructor cannot be called again because it is not saved and is only called to create the object. The purpose of this technique is simply to encapsulate a single complex object code.
Dual-use constructor: new.target
Inside the function, we can check whether its calling method uses the new method. This can be done using a special attribute new.target.
The value is empty when called normally, and the value is the function when called through new.
function User() {
alert(new.target);
}
User(); // undefined
new User(); // function User { ... }The following code can achieve the same effect using the conventional method and the new operation method.
function User(name) {
if (!new.target) { // if you run me without new
return new User(name); // ...I will add new for you
}
this.name = name;
}
let john = User("John"); // redirects call to new User
alert(john.name); // JohnThis method is used in some libraries to make the code more flexible. But it is not a good thing to use it everywhere, because it is not obvious enough for those who are familiar with the internal situation of the User object.
Constructor return value
Usually constructors do not need a return value statement. Its task is to write some necessary content into this object and then automatically return it.
But if there is a return statement, the rule is simple:
- If return returns an object, then this is returned instead of this.
- If return returns the original type, it is ignored and this is still returned.
In other words, return either returns an object or this. The following example returns an object instead of this. The example is as follows:
function BigUser() {
this.name = "John";
return { name: "Godzilla" }; // <-- returns an object
}
alert( new BigUser().name ); // Godzilla, got that objectThis example returns empty or is writing a primitive type. In fact, it does not matter. Influence.
function SmallUser() {
this.name = "John";
return; // finishes the execution, returns this
// ...
}
alert( new SmallUser().name ); // JohnMost constructors do not need to return. This special behavior is reminded here just for the sake of intellectual completeness.
Ignore brackets
By the way, when calling the constructor, if there are no parameters, we can omit the brackets after the object identifier.
let user = new User; // <-- no brackets // same as let user = new User();
Ignoring parentheses is not a good coding style, but it is allowed in the specification.
Methods in constructors
Using constructors to create objects provides great flexibility, and different objects can be defined by providing parameters.
Of course, we not only add attributes to this, but also add methods.
The following example creates an object, uses the name parameter, and adds a method sayHi:
function User(name) {
this.name = name;
this.sayHi = function() {
alert( "My name is: " + this.name );
};
}
let john = new User("John");
john.sayHi(); // My name is: John
/*
john = {
name: "John",
sayHi: function() { ... }
}
*/Summary
The constructor, or constructor for short, is an ordinary Functions, but there is a general convention that the first letter of the name is capitalized.
The constructor is called using new, that is, it starts to create an empty this object, and then returns this with filled content.
We can use the constructor to create multiple similar objects. This is not the only content of the day, which will be further explained later.
Javascript provides constructors for some built-in objects, such as Date, Set, etc.
The above is the detailed content of Detailed explanation of examples of constructor and new operator in JavaScript. For more information, please follow other related articles on the PHP Chinese website!
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
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
1662
14
1419
52
1313
25
1262
29
1236
24
Recommended: Excellent JS open source face detection and recognition project
Apr 03, 2024 am 11:55 AM
Face detection and recognition technology is already a relatively mature and widely used technology. Currently, the most widely used Internet application language is JS. Implementing face detection and recognition on the Web front-end has advantages and disadvantages compared to back-end face recognition. Advantages include reducing network interaction and real-time recognition, which greatly shortens user waiting time and improves user experience; disadvantages include: being limited by model size, the accuracy is also limited. How to use js to implement face detection on the web? In order to implement face recognition on the Web, you need to be familiar with related programming languages and technologies, such as JavaScript, HTML, CSS, WebRTC, etc. At the same time, you also need to master relevant computer vision and artificial intelligence technologies. It is worth noting that due to the design of the Web side
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
Essential tools for stock analysis: Learn the steps to draw candle charts with PHP and JS
Dec 17, 2023 pm 06:55 PM
Essential tools for stock analysis: Learn the steps to draw candle charts in PHP and JS. Specific code examples are required. With the rapid development of the Internet and technology, stock trading has become one of the important ways for many investors. Stock analysis is an important part of investor decision-making, and candle charts are widely used in technical analysis. Learning how to draw candle charts using PHP and JS will provide investors with more intuitive information to help them make better decisions. A candlestick chart is a technical chart that displays stock prices in the form of candlesticks. It shows the stock price
PHP and JS Development Tips: Master the Method of Drawing Stock Candle Charts
Dec 18, 2023 pm 03:39 PM
With the rapid development of Internet finance, stock investment has become the choice of more and more people. In stock trading, candle charts are a commonly used technical analysis method. It can show the changing trend of stock prices and help investors make more accurate decisions. This article will introduce the development skills of PHP and JS, lead readers to understand how to draw stock candle charts, and provide specific code examples. 1. Understanding Stock Candle Charts Before introducing how to draw stock candle charts, we first need to understand what a candle chart is. Candlestick charts were developed by the Japanese
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
The relationship between js and vue
Mar 11, 2024 pm 05:21 PM
The relationship between js and vue: 1. JS as the cornerstone of Web development; 2. The rise of Vue.js as a front-end framework; 3. The complementary relationship between JS and Vue; 4. The practical application of JS and Vue.
How to get HTTP status code in JavaScript the easy way
Jan 05, 2024 pm 01:37 PM
Introduction to the method of obtaining HTTP status code in JavaScript: In front-end development, we often need to deal with the interaction with the back-end interface, and HTTP status code is a very important part of it. Understanding and obtaining HTTP status codes helps us better handle the data returned by the interface. This article will introduce how to use JavaScript to obtain HTTP status codes and provide specific code examples. 1. What is HTTP status code? HTTP status code means that when the browser initiates a request to the server, the service
Learn best practice examples of pointer conversion in Golang
Feb 24, 2024 pm 03:51 PM
Golang is a powerful and efficient programming language that can be used to develop various applications and services. In Golang, pointers are a very important concept, which can help us operate data more flexibly and efficiently. Pointer conversion refers to the process of pointer operations between different types. This article will use specific examples to learn the best practices of pointer conversion in Golang. 1. Basic concepts In Golang, each variable has an address, and the address is the location of the variable in memory.
