When learning JavaScript, it is always difficult to understand the usage of call() and apply(). I believe many friends have the same idea as me. Now share it with your friends.
call
Syntax: call([thisObj[,arg1[, arg2[, [,.argN]]]]])
Definition: Call an object Method to replace the current object with another object.
After reading the syntax and definition, it is still difficult to understand. The call function can accept several parameters. The first parameter is the object of the current operation, and the following parameters are the parameters of the called method. So It’s still hard to understand, so go straight to the code:
function Teacher(name, age) { //定义一个构造函数Teacher this.name = name; this.age = age; this.intro = function() { alert("My name is " + this.name + "," + "I'm " + this.age) } } function Student(name, age) { //定义一个构造函数Student Teacher.call(this, name, age); //Student调用了Teacher的方法 } var studentA = new Student("Lily", 19); //实例化一个studentA,传入参数 studentA.intro(); //调用studentA的intro方法 输出:My name is Lily,I'm 19
In the above example, studentA is instantiated through the constructor Student. It should inherit all the inheritable methods of the Student constructor, but we see that the Student function does not have any method, but Student called it, which can be said to have inherited Teacher's method, so studentA also has the intro() method. The tenth line of the code plays a key role Teacher.call(this, name, age). The function of this sentence is to make the current object call the method of another object. The this keyword here refers to the Student() object. After instantiation this refers to the studentA object, and the following parameters are the parameters passed in by Teacher. Maybe the above example is not very common, but we often see code like this:
var str="你好美女"; Array.prototype.join.call(str,"!"); //你!好!美!女
The join method is a unique method for arrays. It is a method defined in the prototype of the Array() constructor, so we instantiate All arrays that have been transformed inherit this method, but the string str does not have a join method, but here we borrow the join method of the array to process the string. The expression on the left side of call is the method of the borrowed object, and the first parameter object in call() borrows the method of the expression object on the left side of call (I don’t know if I can understand this, so I’ll just make do with it).
apply
The apply() method has the same function as the call method, except that the parameters passed in are different.
Syntax: apply([thisObj[,argArray]])
Definition: Apply a method of a certain object and replace the current object with another object.
The first parameter of apply is the same as call, and the second parameter is an array object (array-like can also be used). It no longer passes parameters in the form of a single parameter list, but Put all parameters into an array and pass them in together. Here we have to talk about the arguments object, which exists in every function and is an array-like object composed of all the actual parameters of the function. So the second parameter of apply is passed in the arguments object. Look at the following example:
var arr=[2,3,12,5,234,199,21,1000,2432]; var max=Math.max.apply(Math,arr); console.log(max); //2432
There is no method to find the maximum value in the array, but there is max in the Math object to find the maximum value, so we use the apply method to let the array inherit the max() method of Math to find the array. The maximum value of The beauty of these two functions goes far beyond that. Experts are welcome to add more.
The above is the detailed content of Understanding call() and apply() in js. For more information, please follow other related articles on the PHP Chinese website!

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.

Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

Different JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.

JavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.


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

Atom editor mac version download
The most popular open source editor

SublimeText3 Linux new version
SublimeText3 Linux latest version

SublimeText3 Mac version
God-level code editing software (SublimeText3)

SublimeText3 English version
Recommended: Win version, supports code prompts!

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.