


JavaScript design patterns factory pattern and constructor pattern_javascript skills
What is a pattern
I was preparing for the final exam a while ago. It was so tiring and tiring, and I really didn’t have time to update the article. Today I will talk to you about the design patterns in JavaScript.
First of all, what we need to know is: a pattern is a reusable solution, while an anti-pattern is a poor solution to a certain problem.
Common examples of js anti-patterns
1. Pass strings to setTimeout and setInterval instead of functions, which triggers the internal use of eval().
2. Define a large number of variables in the global context to pollute the global namespace
3. Modify the prototype of the Object class
4. Use js in inline form. The js code embedded in the HTML file cannot be included in external unit testing tools.
5. Abusing document.write. If document.write is executed after the page is loaded, it will rewrite the page we are on. If you can use document.creatElement instead, try not to use document.write.
Categories of design patterns
Creative Design Pattern
Creative design patterns focus on handling object creation mechanisms to create objects in a way that is appropriate for a given situation. Properties that fall into this category include:
Constructor, Factory, Abstract, Prototype, Singleton and Builder
Structural Design Pattern
Structural patterns are about object composition and can often be used to find simple ways to establish relationships between different objects.
Patterns that fall into this category include:
Decorator, Facade appearance, Flyweight, Adapter adapter and Proxy agent
Behavioral Design Patterns
Behavioral patterns focus on improving or simplifying communication between different objects in the system.
Behavior patterns include:
Iterator, Mediator, Observer and Visitor
Factory mode
In order to solve the problem of multiple similar object declarations, we can use a method called factory pattern. This method is to solve the problem of a large number of duplications when instantiating objects.
Classification of Factory Pattern
Factory mode is divided into simple factory, abstract factory and smart factory. Factory mode does not require the use of a constructor explicitly.
Simple Factory Pattern: Use a class (usually a singleton) to generate instances.
Complex factory pattern: Use subclasses to determine which specific class instance a member variable should be.
Benefits of Factory Model
The main benefit is that you can eliminate the coupling between objects by using engineering methods instead of the new keyword. Prevent code duplication by centralizing all instantiation code in one location.
Disadvantages of factory model
For most classes, it is best to use the new keyword and constructor, which can make the code simpler and easier to read. Rather than having to look at the factory method to find out.
The factory pattern solves the problem of repeated instantiation, but there is another problem, that is, the problem of identification, because it is impossible to figure out which object they are instances of.
alert(typeof test1); //Object
alert(test1 instanceof Object); //true
When to use factory pattern?
Factory mode is mainly used in the following scenarios:
1. When objects or components involve high complexity
2. When you need to easily generate different instances of an object according to different environments
3. When dealing with many small objects or components that share the same properties
Constructor pattern
Constructor (constructor method) can be used in ECMAScript to create specific objects. This mode can just solve the problem that the above factory mode cannot identify object instances.
Using the constructor method not only solves the problem of repeated instantiation, but also solves the problem of object recognition. The difference between this mode and the factory mode is:
1. The constructor method does not create an object explicitly (new Object());
2. Directly assign properties and methods to this object;
3. There is no renturn statement.
There are some specifications for constructor methods:
1. The function name and the instantiation constructor name are the same and capitalized (PS: It is not mandatory, but writing this way helps distinguish the constructor from the ordinary function);
2. To create an object through a constructor, you must use the new operator.
Since objects can be created through constructors, where does this object come from, and where is new Object() executed? The execution process is as follows:
1. When a constructor is used and new constructor() is used, new Object() is executed in the background;
2. Give the scope of the constructor to the new object (that is, the object created by new Object()), and this in the function body represents the object created by new Object().
3. Execute the code within the constructor;
4. Return the new object (return directly in the background).
Constructor with prototype
There is an attribute called prototype in js. After calling the js constructor to create an object, the new object will have all the properties of the constructor prototype. In this way, multiple Car objects can be created and have access to the same prototype.
Now a single instance of run() can be shared among all Car objects.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.


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