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问题:

I know this question was asked many times, but I haven't found answer. So why its recommended to include scripts at the end of body tag for better rendering?

From Udacity course https://www.udacity.com/course/ud884 - rendering starts after DOM and CSSOM are ready. JS is HTML parse blocking and any script starts after CSSOM is ready.

So if we got:

<html>
    <head>
        <meta name="viewport" content="width=device-width,initial-scale=1">
        <title>CRP</title>
        <link rel="stylesheet" href="styles.css">
    </head>
    <body>
        <!-- content -->
        <script src="script.js"></script>
    </body>
</html>

CRP would be:

CSSOM ready > JS execute > DOM ready > Rendering

And if script is at head:

<html>
    <head>
        <meta name="viewport" content="width=device-width,initial-scale=1">
        <title>CRP</title>
        <link rel="stylesheet" href="styles.css">
        <script src="script.js"></script>
    </head>
    <body>
        <!-- content -->
    </body>
</html>

CRP would be the same:

CSSOM ready > JS execute > DOM ready > Rendering

This question is only about "sync" scripts (without async/defer attribute).

回答1:

Scripts, historically, blocked additional resources from being downloaded more quickly. By placing them at the bottom, your style, content, and media could download more quickly giving the perception of improved performance.

Further reading: The async and defer attributes.

回答2:

In my opinion, this is an outdated practice. More recently, the preference is for JavaScript to separate any code that requires the DOM to be present into a "DOMContentLoaded" event listener. This isn't necessarily all logic; lots of code can initialize without access to the complete DOM.

It's true that this causes a small moment when only the script file is being retrieved, and nothing else (for instance, images). This small window can be skipped by adding the async attribute, but even without it I recommend putting script tags in the head so that the browser knows as soon as possible to load them, rather than saving them (and any future JS-initiated requests) for last.

回答3:

Images placed below the script tag will wait to load until the JS script loads. By placing the script tag at the bottom you load images first, giving the appearance of a faster page load.

回答4:

I think it depends on your website or app. Some web apps are based on JavaScript. Then it does not make sense to include it at the bottom of the page, but load it immediately. If JavaScript just adds some not so important features to some content based page, then better load it at the end. Loading time will almost be the same, but the user will see the important parts earlier (before the page finished loading).

It’s not about a whole site loading faster, but giving a user the impression of some website loading faster.

For example: This is why Ajax based websites can give a much faster impression. The interface is always the same. Just some content parts will alter.