The steps and principles of browser access to web pages JAVASCRIPT

Browser access web page steps

Modern browsers are built from a series of basic technologies and components, mainly including the following aspects:

1. User interface: used to display web page content, browser toolbar, address bar, etc.

2. Layout engine: used to parse HTML and CSS files, and perform page structure layout and style typesetting.

3. JavaScript engine: used to parse and execute JavaScript codes to achieve interactive effects and dynamic effects on web pages.

4. Rendering engine: used to render page content and graphic images into an interactive user interface.

5. Network engine: used to process network requests and responses, and realize communication between the browser and the server.

When a user starts a browser and enters a URL address, the browser parses the URL, constructs an HTTP request, and sends the request to the server. After receiving the request, the server will encapsulate the corresponding HTML file and other resources, such as JavaScript and CSS files, into an HTTP response message and send it back to the browser. After the browser receives the response message, it will parse the HTML file and load the JavaScript file in it.

The specific process for the browser to run JavaScript is as follows:

1. The browser parses the JavaScript code into an Abstract Syntax Tree (AST) and performs syntax and semantic checks.

2. A parser compiles JavaScript code into bytecode, which then runs the program through a bytecode interpreter.

3. While executing the bytecode, the JavaScript engine will implement operations such as function calls, variable access, and object access based on the execution context and scope in the code, as well as the mechanism of the execution stack.

4. When JavaScript code needs to operate browser DOM elements or perform Ajax requests and other operations, it will call the Web API interface provided by the browser, such as document, XMLHttpRequest, fetch, etc., to realize these functions.

5. Web API encapsulates the browser's own underlying methods and system services, and exposes callable APIs to JavaScript code, enabling JavaScript to interact with the underlying system.

6. After the Web API call is completed, the JavaScript engine will respond to the interface request, continue to execute the code of the next step, and repeat the above operations until the execution of the JavaScript code ends.

Through this process, the browser can parse and execute the JavaScript code, and finally render the operation result to the user interface, so as to realize the dynamic effect and interactive characteristics of the web page content.

Debugging process

To enter the debugging page in the browser, you can use the developer tools provided by the browser. The developer tools of different browsers are different, but usually you can use the F12 shortcut key or right-click on a blank space in the web page, and select options such as "Inspect" or "Inspect Element" to open the corresponding debugging page.

After entering the developer tool, you can view the HTTP request sent by the browser to the server and the HTTP response returned by the server. Specific steps are as follows:

1. Open the developer tools and switch to the "Network" or "Network" tab.

2. Refresh the webpage, observe the request list in the "Network" panel, and you can see all the HTTP requests sent by the browser to the server, including the requested URL, method, header information, request parameters, etc.

3. Click on a request to view the detailed information of the request, including request and response header information, response content, response status code, etc.

4. In this detailed information page, you can view all information about HTTP requests and responses, including request URL, request method, request header information, request parameters, response header information, response content, response status code, etc.

5. In the HTTP request and response information, you can view all the details of the communication between the server and the browser, including protocol version, encryption method, encoding format, etc.

These steps make it easy to view the HTTP requests that the browser sends to the server and the HTTP responses that the server returns in order to diagnose and debug web page performance and functionality issues. In addition, developer tools also provide functions such as debugging JavaScript code, viewing DOM trees, style sheets, etc., which can help developers better understand the structure and behavior of web pages.

JavaScript

JavaScript is a scripting language that typically runs in web browsers and is used to add interactivity and animation to web pages. When a browser loads a web page, it loads JavaScript code at the same time, and executes these codes when the user interacts with the web page, thereby realizing the interaction between the user and the web page.

However, the execution of JavaScript is not limited to browsers, and JavaScript code can also be run in the Node.js environment. Node.js is an open source, cross-platform JavaScript runtime environment that allows JavaScript code to run on the server side. Node.js enables developers to use JavaScript to build back-end applications such as web applications, APIs, tools, and more.

Therefore, JavaScript can run either in the browser or in the Node.js environment, depending on the specific application scenarios and needs.

browser

Running in the browser refers to the script code written in the JavaScript language, which is embedded in webpage files such as HTML and CSS. When the user opens the webpage in the browser, the browser will parse the webpage file and run JavaScript at the same time Code to realize functions such as dynamic effects, interactive behavior and data processing of web pages.

A browser is a software program that runs on a client (user device) and is used to display and interpret web page content. When a user enters a URL (website address) in a browser, the browser will send a request to the server to obtain the HTML, CSS, JavaScript and other files of the web page, then parse these files and display them on the user's device in a corresponding manner web content. The browser can run on various operating systems such as Windows, macOS, and Linux, and includes various browser software such as Chrome, Firefox, Edge, and Safari.

In simple terms, JavaScript code runs in a browser, a client-side application that is installed on a user's device and used to display and interpret web page content.

Browsers, operating systems, hardware platforms, and programming languages ​​are four different objects, and there are some connections and dependencies between them.

Browsers, operating systems, hardware platforms, and programming languages ​​are four different objects, and there are some connections and dependencies between them.

• Browser: A client-side application that displays and interprets web page content. Browsers usually parse files such as HTML, CSS, and JavaScript codes into web pages and display them on the user's device. Browsers rely on the infrastructure and resources provided by the operating system and hardware platform, such as network connections, memory, CPU, and so on.

• Operating System: A collection of programs that control a computer's hardware and software resources. The operating system is usually responsible for the management and coordination of basic computer functions, such as memory management, process management, file system management, and so on. The browser needs to run on the operating system in order to obtain the required basic services such as network and graphics.

• Hardware platform: refers to the composition and design of computer hardware. It includes various hardware components such as CPU, memory, hard disk, graphics card, motherboard, etc. Understanding the characteristics of the hardware platform can have an impact on the behavior of software development. For example, the browser needs to consider the characteristics of the hardware platform, optimize some performance bottlenecks, and improve user experience.

• Programming language: It is a language used for programming, which usually includes some grammar rules, compiler/interpreter, etc. Programming languages ​​can implement a wide variety of applications and systems. JavaScript in the browser is a programming language, which is used to achieve dynamic effects, interactive behavior, data processing, etc. JavaScript depends on the support of the browser environment and operating system environment.

Collectively, the above objects are interdependent and create complex relationships among them, creating infrastructure, platforms, and software. The browser needs to run on the operating system, depends on the resources and services provided by the operating system, and relies on the acceleration means and high-speed memory provided by the hardware platform. JavaScript relies on the support of the browser and operating system environment, and needs to make good use of computer resources to cope with complex and changeable application requirements.

After you describe a webpage, what is the data interaction flow in the middle?

Data interaction in a web page usually involves the following steps:

1. The client sends a request: When the user enters a URL in the browser or clicks on a related link, the browser will send an HTTP request to the server to request the corresponding web page or data.

2. Server response request: After receiving the request, the server makes corresponding processing according to the content of the request, and generates response content, including HTML, CSS, JavaScript, pictures, videos and other resources. The server packages these resources into an HTTP response and sends it back to the client.

3. Client-side rendering: When the browser receives the response returned by the server, it will start to parse the response content, render according to the HTML, CSS, JavaScript code and other information in the response, generate a web interface and present it to the user.

4. Data interaction: When data interaction is required in a web page, such as when a user submits a form or clicks, the browser will capture the relevant event and process it through JavaScript code.

5. Client sends request: In JavaScript code, tools such as XMLHttpRequest or Fetch can usually be used to send requests to the server through the client to obtain relevant data or responses.

6. The server responds to the request: After receiving the request sent by the client, the server makes corresponding processing according to the content of the request, and returns the corresponding data or response.

7. Client update page: According to the response returned by the server, the client can use JavaScript to update the page content, such as dynamically adding, deleting or modifying page elements to achieve data display and interaction effects.

In this way, in a web page, data interaction can be realized through multiple stages such as client sending a request, server responding to the request, client rendering and updating. Among them, JavaScript code can act as a bridge to connect the front-end interface and back-end services to realize data transfer and interaction.

The front end written in the language, after running, there will be a website that can be opened in the browser. What is the principle?

A front-end program written in a language is usually composed of three parts: HTML, CSS, and JavaScript. The combination of these three languages ​​can realize the rendering and interaction effects of web pages.

When this front-end program is written, it can be served by a Web server. The web server usually provides a specific URL, and the user can access the front-end program by entering this URL. When the user enters a URL in the browser, the browser will send an HTTP request to the Web server to request the corresponding front-end program.

When the web server receives the HTTP request, it will send the request to the application backend. After receiving the request, the application backend will perform corresponding processing according to the URL corresponding to the request, and return the processing result to the web server. After receiving the results returned by the backend of the application, the web server returns the HTML, CSS and JavaScript files of the front-end program to the browser. The browser renders the layout, structure and style of the webpage according to the content of the HTML file, and realizes the interactive and dynamic effects of the page according to the JavaScript code.

When the user clicks a link or submits a form in the browser, the browser will capture the corresponding event, process it according to the JavaScript code, construct a new HTTP request and send it to the web server, and execute it in the application backend The corresponding processing returns the response result, which is then presented and displayed by the front-end program.

Therefore, through such a process, the front-end program can generate a corresponding web page, and the user can open and access this web page in a browser.