JavaScript learning summary [1] First introduction to JS

1. What is JavaScript?

JavaScript is a cross-platform, object-oriented, dynamic, weakly typed, lightweight interpreted language. It is a client-side scripting language that is object- and event-driven and has relative security. Applied to HTML documents, it can provide dynamic interaction capabilities on the website, which is different from Java. Simply put, it processes HTML documents based on the browser, implements various web page special effects, responds to various user operations, adds dynamic effects to the web page, and improves the user operating experience, such as picture scrolling playback effects, clicking the login button to pop up a dialog box, and moving the mouse in and out. Animation, form submission data validation, etc.

2. Why should you learn JavaScript?

JavaScript is often called JS. The purpose of its invention is to serve as a built-in scripting language for browsers, providing web developers with the ability to control browsers. It can make web pages present various special effects, providing Provide user-friendly interactive experience. With the emergence of Ajax technology, the front-end can exchange data with the back-end and update page data without refreshing the page. The popularity of jQuery and other libraries has made JS writing extremely simple, and the Bootstrap framework has infinitely reduced the cost of the front-end. It greatly improves the efficiency of front-end development, and JS has a very bright future in the front-end field.

With the release of Node, JS can not only run on the front end, but also on the server. This is a qualitative breakthrough for JS. The Node.js project allows JS to be used to develop large server-side projects. It has become a reality that both the front and back ends of the website are developed with JS.

At this point, in addition to being parsed by browsers, JS can also be used as a back-end language. More and more applications use JS as an embedded scripting language, which can be used to build mobile APPs and develop HTML. Games can be built as desktop applications without relying on a browser.

It can be expected that in the end, using only JS, you can develop programs that adapt to different platforms (including desktop, server, and handheld). In a blog posted by Jeef Atwood, he proposed the famous "Atwood's Law", that is, "Any application that can be implemented in JavaScript will eventually be implemented in JavaScript."

Compared to learning other languages, JS is easy to learn. As long as you have a browser, you can run JS programs, and as long as you have a text editor, you can write JS code. No need to install complex IEDs (Integrated Development Environments) and compilers. The syntax of JS is relatively simple. The syntax itself is not particularly rich, and the language is flexible. You can complete most operations with only simple commands.

Although the core syntax of JS is not difficult to learn, it is not easy to truly learn it thoroughly. JS is actually very complicated. As you learn, you will become more and more aware of its power. For JS to work, it must cooperate with other components. These external components are diverse and huge in number, involving all aspects of network applications, such as editor components, follow components provided by QQ space, etc. It is not easy to master them and must be downloaded. Hard work. The JS language has some design flaws, and weird running results will occur in some places. Mainstream browsers have different support for JS. Compatibility is the most troublesome thing. Learning JS requires a large part of your time. Find out where there are traps.

Despite this, JS’s status is still unshakable. In the world language usage rankings released in 2015, JS ranked seventh, an improvement compared to last year. Ecma (which sets the standard for scripting languages ​​through Ecma-262) has accelerated the standardization of the language, making JS increasingly more powerful, while grammatical defects and weirdness have also been compensated. As of this year, the latest version of JS is ECMAScript 2015, also called ES6, which has been added many new features. Therefore, JS is still worth learning, not only to master it, but also to learn it well, especially for Web front-end development engineers.

3. Composition of JavaScript

JS is composed of three parts:

ECMAScript: also called an interpreter, acting as a translator, this is the core of JS part.

DOM: Document Object Model. DOM gives JS the ability to operate HTML, that is, document operation.

BOM: Browser Object Model. BOM gives JS the ability to operate the browser, that is, window operation.

4. JavaScript usage

Scripts in HTML must be located between tags. You can place an unlimited number of scripts in an HTML document. The script can be in the or of the HTML, or both. The usual approach is to place the function in or at the bottom of the page so that it does not interfere with the content of the page.

　You can also save the script in an external file with a .js extension. External files usually contain code used by multiple web pages. Note: When using external scripts, the