Since the browser is single-threaded, the script will block the download of other resources when loading; although the browser has been improved now, it still needs to be improved.
Obviously, the scripts must be executed in order, but there is no need to download them in order. Solution:
1. Embedded JS
Usually because page size and caching can bring more benefits, it is better to introduce JS into external files;
In a few cases, such as the homepage and a small amount of JS, it is acceptable.
2. XHR Eval
Get the script from the server side via XMLHttpRequest.
The main drawback is that the script obtained through XHR must be deployed in the same domain as the main page.
Ajax.get("test.js", function ( xhr) {
eval(xhr.responseText);
});
3. XHR Injection
Use XHR to get the script and create script tags.
Similarly, scripts obtained via XHR must be deployed in the same domain as the main page.
Ajax.get('test.js', function ( xhr) {
injectscript(xhr.responseText);
});
function injectscript(scriptText) {
var s = document.createElement('script');
s.text = scriptText;
document.getElementsByTagName('head')[0].appendChild(s);
}
4. Script in Iframe
Put the required script into a page, and then load the page through iframe.
The disadvantage is that the cost of the iframe itself is relatively large, and the browser security mechanism does not allow js in the iframe to access the cross-domain parent page, and vice versa.
5. Script DOM Element
JS dynamically creates a script DOM element and sets its src attribute.
var scriptElem = document.createElement('script');
scriptElem.src = 'http://domain.com/test.js';
document.ge('head')[0].appendChild(scriptElem);
6. Script Defer
Add the defer attribute to the script tag.
The disadvantage is that it is only supported by IE and some new browsers.
7. document.write Script Tag
Use document.write to write the HTML tag script into the page.
The disadvantage is that scripts are loaded in parallel only in IE.
document.write("");
One difference that has not been widely discussed is the impact on the browser’s busy state, including the browser’s status bar, progress bar, tab icon, and mouse.
When you load multiple scripts that have dependencies on each other, you also need a technology that can guarantee the order of execution.
技术
|
并行下载
|
可以跨域
|
存在Script标签
|
忙碌指示 |
顺序保证 |
大小 (bytes) |
XHR Eval |
IE, FF, Saf, Chr, Op |
no |
no |
Saf, Chr |
- |
~500 |
XHR Injection |
IE, FF, Saf, Chr, Op |
no |
yes |
Saf, Chr |
- |
~500 |
Script in Iframe |
IE, FF, Saf, Chr, Op |
no |
no |
IE, FF, Saf, Chr |
- |
~50 |
Script DOM Element |
IE, FF, Saf, Chr, Op |
yes |
yes |
FF, Saf, Chr |
FF, Op |
~200 |
Script Defer |
IE, Saf4, Chr2, FF3.1 |
yes |
yes |
IE, FF, Saf, Chr, Op |
IE, FF, Saf, Chr, Op |
~50 |
document.write Script Tag |
IE, Saf4, Chr2, Op |
yes |
yes |
IE, FF, Saf, Chr, Op |
IE, FF, Saf, Chr, Op |
~100 |
Currently there are several methods to maintain the execution order during asynchronous loading. Due to space reasons, they will not be described in detail.
Single Script
1, Hardcoded Callback
2, Window Onload
3, Timer
4, Script Onload
5, Degrading Script Tags
Multiple Scripts
1 , Managed XHR
2. DOM Element and Doc Write
This article refers to the "Advanced Guide to High-Performance Website Construction"

The differences in performance and efficiency between Python and JavaScript are mainly reflected in: 1) As an interpreted language, Python runs slowly but has high development efficiency and is suitable for rapid prototype development; 2) JavaScript is limited to single thread in the browser, but multi-threading and asynchronous I/O can be used to improve performance in Node.js, and both have advantages in actual projects.

JavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.

JavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.


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