How does javascript run?

JavaScript is a scripting language commonly used for web development. In this article, we will explore how JavaScript code runs.

JavaScript code can run in a variety of environments, including browsers, desktop applications such as Node.js and Electron. We will focus on the process of running JavaScript in the browser.

When a web page is loaded, the browser parses the HTML and CSS code and creates the DOM and CSSOM object model. Once the process is completed, the browser starts executing the JavaScript code.

First, the browser will create a global execution context (Global Execution Context), which is the starting point for JavaScript code execution. The global execution context is the top context of JavaScript code, which contains the global object (window object) and some global functions and variables.

Once the global execution context is established, the browser will begin to parse and execute JavaScript code in the order of the code. For each line of JavaScript code, the browser creates an execution context (Execution Context).

The execution context contains all the information needed for this line of code, including variables, functions, and the current object. Each execution context is added to the Execution Context Stack.

The execution process of JavaScript code is based on the event loop (Event Loop). The event loop switches between each execution context while listening for events in the event queue (Event Queue). On each iteration of the event loop, the event loop takes the next event from the event queue and adds it to the execution stack as a new execution context. Once that execution context is complete, the event loop again fetches the next event from the event queue and repeats the process.

The execution process of JavaScript code depends on the structure of the code itself, as well as the functions and APIs called when executing the code. During the execution of code, execution contexts in the execution context stack may be suspended, nested, terminated, etc.

The execution process of JavaScript code is also affected by some other factors, such as asynchronous code and event handlers. Asynchronous code runs in the background without blocking the main thread. Event handlers are typically used to respond to user interaction events and add them to the event queue for processing.

In short, the process of running JavaScript in the browser is a complex process, but it is the key that allows us to write interactive and dynamic web pages. By gaining a deep understanding of how JavaScript executes, we can better optimize our code and write more efficient applications.

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