How come cheerio $ variable doesn't affect to other sessions?

In the Node.js cheerio library, the cheerio$ variable is a common naming convention used to reference the instance created after loading HTML with cheerio. This instance allows us to manipulate the loaded HTML as we would with jQuery. The reason the cheerio$ variable does not affect other sessions lies primarily in Node.js's execution mechanism and cheerio's design philosophy.

1. Node.js's Isolated Scope

Node.js executes each request in an isolated scope. This means variables created within a session, such as cheerio$, are only valid within that session's scope. Even for concurrent requests, each request has its own scope and variable instance, so a cheerio$ variable in one session does not interfere with other sessions.

2. Cheerio's Statelessness

Cheerio is designed to be stateless, meaning it does not store any information about parsed HTML or DOM state. When you create a new instance using cheerio.load(html), it is completely independent. This ensures that each call to the load method creates a brand new, unrelated cheerio$ instance.

3. Instance Independence

Each time you use cheerio.load(html) to load HTML, it returns a new cheerio$ instance. This instance only contains the data and methods for the currently loaded HTML document. Therefore, even with multiple concurrent requests, each request processes its own HTML document and operations independently.

Practical Example

Suppose we use cheerio on a web server to handle web scraping requests from different users. Each user's requested webpage content may differ, so we call cheerio.load(html) for each request as follows:

javascript
app.get('/process', (req, res) => {
    const html = fetchHtmlFromUrl(req.query.url);  // Assume this function fetches HTML based on the request URL
    const cheerio$ = cheerio.load(html);  // Create a new cheerio instance for this request

    // Use cheerio$ to process HTML
    const title = cheerio$('title').text();

    res.send(`Title of the page is: ${title}`);
});

In this example, each user request creates an independent cheerio$ instance, ensuring that requests from different users are isolated and do not interfere with each other.

In summary, the cheerio$ variable does not affect other sessions primarily due to Node.js's scope isolation and cheerio's stateless design, where each instance is independent and self-contained.