HTNL introduction

Let's see JavaScript What's special , What can we do with it , And what other technologies can work with it .

What is? JavaScript？

JavaScript Originally created for “ Make the web page more vivid ”.

The program written in this programming language is called Script . They can be written directly on the page HTML in , Automatically execute when the page is loaded .

Scripts are provided and executed in plain text . They don't need special preparation or compilation to run .

This aspect ,JavaScript and Java There's a big difference .

Why call JavaScript？

JavaScript When it was first born , Its name is “LiveScript”. But because at that time Java Is very popular , So I decided to position a new language as Java Of “ Younger brother ” It will help its popularity .

With JavaScript The development of , It has become a completely independent language , And also has its own language norms ECMAScript. Now? , It and Java There is no relationship between .

Now ,JavaScript Not only in the browser , It can also be executed on the server , It can even be carried anywhere JavaScript engine Executed in the device .

Embedded in the browser JavaScript engine , Sometimes called “JavaScript virtual machine ”.

Different engines have different “ Code name ”, for example ：

• V8) —— Chrome、Opera and Edge Medium JavaScript engine .
• SpiderMonkey —— Firefox Medium JavaScript engine .
• …… There are other codes , image “Chakra” be used for IE,“JavaScriptCore”、“Nitro” and “SquirrelFish” be used for Safari, wait .

These terms are easy to remember , Because they often appear in developer articles . We also use these terms . for example , If “V8 Support a function ”, So we can think that this function can be in Chrome、Opera and Edge Normal operation in .

How the engine works ？

The engine is complicated , But the fundamentals are simple .

1. engine （ If it's a browser , Then the engine is embedded ） Read （“ analysis ”） Script .
2. then , The engine transforms the script into （“ compile ”） For machine language .
3. then , Machine code executes quickly .

The engine optimizes every stage of the process . It can even monitor compiled scripts as they run , Analyze the data flowing through the script , And further optimize the machine code according to the obtained information .

In the browser JavaScript What can be done ？

modern JavaScript It's a kind of “ Safe ” programing language . It does not provide for memory or CPU Bottom level access to , Because it was originally created for browsers , You don't need these functions .

JavaScript The ability of the depends largely on the environment in which it operates . for example ,Node.js Support allows JavaScript Read / Write to any file , Functions that execute network requests, etc .

In the browser JavaScript Can do with webpage operation 、 User interaction and Web Everything about servers .

for example , In the browser JavaScript You can do the following ：

• Add a new HTML, Modify the existing content of the web page and the style of the web page .
• Respond to user behavior , Respond to mouse clicks , The movement of the pointer , Key press .
• Send network requests to remote servers , Download and upload files （ So-called AJAX) and [COMET]
• Get or set cookie, Ask questions or send messages to visitors .
• Remember the client's data （“ The local store ”）.

In the browser JavaScript Nothing to do ？

For users （ Information ） Security , In the browser JavaScript My ability is limited . The purpose is to prevent malicious web pages from obtaining users' private information or damaging users' data .

Examples of such restrictions include ：

• In the web page JavaScript Cannot read 、 Write 、 Copy and execute any file on the hard disk . It doesn't have direct access to the operating system functions .

  Modern browsers allow JavaScript Do some file related operations , But this operation is limited . Only if the user makes a specific behavior ,JavaScript To operate this file . for example , The user puts the file “ Drag and drop ” Go to browser , Or by <input> The tag selects the file .

  There are many with cameras / How the microphone interacts with other devices , But these all need the explicit permission of the user . therefore , To enable the JavaScript The web page should not be secretly activated by webcam to watch you , And send your message to National security agency .

• Different tabs / Windows usually don't know each other . occasionally , There will also be some connections , For example, a tab passes through JavaScript Open another tab . But even in this case , If two tabs don't open the same website （ domain name 、 Any website with different protocol or port ）, They can't communicate with each other .

  That's what's called “ The same-origin policy ”. In order to solve “ The same-origin policy ” problem , Both tabs must be all Contains some specific... That deal with this problem JavaScript Code , And all allow data exchange . This tutorial will cover this part of related knowledge .

  This restriction is also for the information security of users . for example , User opened http://anysite.com Web pages must not be accessible http://gmail.com（ Another tab opens the page ） You can't steal information from there .

• JavaScript You can easily communicate with the server where the current page is located through the Internet . But from other sites / The ability to receive data in the domain's servers is weakened . Even though you can , But you need a clear protocol from the remote server （ stay HTTP header in ）. This is also for the information security of users .

If you are outside the browser environment （ For example, on the server ） Use JavaScript, There is no such restriction . Modern browsers also allow you to install plug-ins that may require extended permissions / Expand .

What makes JavaScript different ？

There are at least 3 One thing is worth mentioning ：

• And HTML/CSS Fully integrated .
• Simple things , Simply do it .
• Supported by all major browsers , And on by default .

JavaScript Is the only browser technology that combines these three things .

That's why JavaScript different . This is why it is the most widely used tool for creating browser interfaces .

Besides ,JavaScript It can also be used to create server and mobile applications .

JavaScript “ The upper ” Language

Different people want different functions .JavaScript The grammar of can't meet everyone's needs .

This is normal , Because everyone's projects and needs are different .

therefore , Many new languages have emerged recently , Before these languages are executed in the browser , Will be compile （ conversion ） become JavaScript.

Modern tools make compilation very fast and transparent , In fact, developers are allowed to write code in another language and will “ Automatic conversion ” by JavaScript.

Examples of such languages are ：

• CoffeeScript yes JavaScript A kind of grammar sugar . It introduces a shorter Syntax , So that we can write more clear and concise code . Usually ,Ruby Developers love it .
• TypeScript Focus on adding “ Strict data types ” To simplify development , To better support the development of complex systems . Developed by Microsoft .
• Flow Also added data types , But in a different way . from Facebook Development .
• Dart It's an independent language . It has its own engine , The engine can run in a non browser environment （ For example, mobile applications ）, It can also be compiled into JavaScript. from Google Development .
• Brython It's a Python To JavaScript The translator of , Let's not use JavaScript Under the circumstances , To be pure Python Write applications .
• Kotlin It's a modern 、 A simple and secure programming language , The application can be written in the browser and Node Running in the environment .

There are many such languages . Of course , Even if we are using such a compilation language , We also need to understand JavaScript. Because I understand JavaScript So that we can really understand what we are doing .

summary

• JavaScript At first, it was a language specially designed for browsers , But now it's also used in many other environments .
• JavaScript As the most widely used browser language , And with the HTML/CSS Fully integrated , Have a unique position .
• There are many other languages that can be “ compile ” become JavaScript, These languages also provide more functions . It's best to understand these languages , At least I have mastered JavaScript Then we'll have a general understanding of .