How does Javascript work? Where to start?

Javascript is a commonly used programming language that can implement interactive functions in web pages, such as form validation, page rendering, etc.

So, how does Javascript run? Where does it start?

First of all, we need to know that Javascript is a client-side scripting language, that is, it runs in the user's browser. When a user opens a web page, the browser automatically downloads the Javascript code in the web page and executes the code during the user's interaction with the web page.

However, the startup of Javascript does not start after all elements in the web page are loaded. In fact, Javascript is executed during the rendering process of the web page.

When a user opens a webpage, the browser will first make a request to the webpage server and download the HTML code of the webpage. The browser then parses the HTML code and renders the individual elements of the page, usually in the order in the HTML code. During this process, if the browser finds that the web page contains Javascript code, it will pause rendering, download the Javascript code to the local, and start executing these Javascript codes after parsing the HTML code of the web page.

Specifically, the startup sequence of Javascript is as follows:

1. First, the browser will parse the HTML code into a DOM tree and a CSSOM tree, and then merge them into a rendering tree .
2. During this process, if Javascript code is encountered, the browser will pause rendering, download the Javascript code to the local, and execute these codes.
3. If the Javascript code contains the document.write statement, it will insert new content in the HTML container and continue to render the web page after all Javascript codes are executed.
4. After the Javascript is executed, the browser will continue to render the web page and layout and draw each element in the page.

It should be noted that the execution of Javascript is single-threaded. That is to say, at the same time, only one Javascript code block can be executed, while other Javascript code blocks will be temporarily placed in the queue waiting for execution. This is called the "event loop".

Therefore, if the Javascript code is too complex or takes too long to execute, it may cause thread blocking, causing users to experience lags when browsing web pages. In order to solve this problem, developers can use asynchronous programming and other technologies to make Javascript code execute at the appropriate time and reduce the burden on the browser.

In short, Javascript is started during the rendering process of the web page. It is a client-side scripting language that can realize the interactive function of the web page. Developers need to pay attention to issues such as Javascript execution order and thread blocking to ensure the performance and user experience of web pages.

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