


The execution process of javascript in the browser (picture and text)_javascript skills
1. The components of most browsers are as shown in the figure
(1) Network - used to complete network calls, such as http requests. It has a platform-independent interface and can work on different platforms
(2) UI backend - used to draw similar combination selections Basic components such as boxes and dialog boxes have a common interface that is not specific to a certain platform. The bottom layer uses the user interface of the operating system
(3) JS interpreter - used to interpret and execute JS code
ps: The above pictures and knowledge points are mainly from "HOW BROWSERS WORK: BEHIND THE SCENES OF MODERN WEB BROWSERS". Students who want to know more about it can focus on it.
2. Most browsers (such as Chrome) use a single thread to execute javascript and update the user interface. This thread, often called the "browser UI thread", can only perform one of these operations at a time, which means that the user interface cannot respond to input while the Javascript code is executing, and vice versa. This is done because the function of the javascript code is to operate the DOM and update the user interface. It can be more efficient to use the same thread to do these two things
3. The work of the browser UI thread is based on a simple queue system, and the tasks will be Save to queue until process is idle. Once free, the next task in the queue is re-extracted and run. These tasks either run JavaScript code or perform UI updates, including redrawing and reflowing.
4. It is important to emphasize that JavaScript runs in a single thread. Don’t be confused by functions such as setTimeout() and setInterVal() and mistakenly think that it is multi-threaded.
ok, the basic points have been explained, let us get to the point and explain the execution process of javascript in the browser.
1. Principle
Generally speaking, every time the <script> tag appears, it will overbearingly make the page wait for the parsing and execution of the script. Regardless of whether the current Javascript is embedded or contains an external link file, the page Both downloading and rendering must stop and wait for script execution to complete. This is necessary in the life cycle of the page, because the page content may be modified during script execution. A typical example is using document.write() in the page. <BR>When the javascript code is embedded in html, this is relatively easy to understand, but when the javascript is an external link file, it is a little bit more loaded, because there is a loading process, and the browser often loads the js file. Cache it. <BR>First, we use the following example to illustrate the caching problem<BR><div class="codetitle"><span><a style="CURSOR: pointer" data="13076" class="copybut" id="copybut13076" onclick="doCopy('code13076')"><U>Copy the code The code is as follows:<div class="codebody" id="code13076"> <BR><html> <BR><head> <BR><script type='text/javascript' src='js/f2.js'></script>
> ;

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.


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