Introduction

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このチャプターではJavaScriptの紹介し、いくつかの基本的なコンセプトについて議論します。

事前に知っておくべきこと

このガイドは、以下の基本的なバックグラウンドを知っている前提となっております。:

  • インターネットとWorld Wide Web(WWW)についての一般的な理解
  • HyperText Markup Language(HTML)についての実用的な知識
  • プログラミング経験。プログラミングが初めての方は、JavaScriptについてのメインページにあるチュートリアルをお試しください。

JavaScriptの情報源

MDNにあるJavaScriptのドキュメントは以下に含まれます。:

  • Webについて学ぶ では初心者の方に向けた情報を提供しており、プログラミングとインターネットの基本的なコンセプトを提供しております。
  • JavaScript ガイド (このガイドページです) ではJavaScript言語とその目的に関する概要を提供しております。 
  • JavaScript Reference はJavaScriptに関する詳細なリファレンスマニュアルを提供しております。

もしあなたがJavaScriptを初めて学ぶ場合、 learning area および JavaScript ガイド の記事から開始してください。基礎をしっかり学んだら、個別のobjectやstatementについての詳細を知るために JavaScript リファレンス を使うことができます。

JavaScriptとは?

JavaScriptはクロスプラットフォームでオブジェクト指向なスクリプト言語です。小さく軽量な言語です。ホスト環境内では、JavaScriptはプログラマティックなコントロールを提供するための環境のオブジェクトに接続することができます。

JavaScript contains a standard library of objects, such as Array, Date, and Math, and a core set of language elements such as operators, control structures, and statements. Core JavaScript can be extended for a variety of purposes by supplementing it with additional objects; for example: 

  • Client-side JavaScript extends the core language by supplying objects to control a browser and its Document Object Model (DOM). For example, client-side extensions allow an application to place elements on an HTML form and respond to user events such as mouse clicks, form input, and page navigation.
  • Server-side JavaScript extends the core language by supplying objects relevant to running JavaScript on a server. For example, server-side extensions allow an application to communicate with a database, provide continuity of information from one invocation to another of the application, or perform file manipulations on a server.

JavaScript and Java

JavaScript and Java are similar in some ways but fundamentally different in some others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript follows most Java expression syntax, naming conventions and basic control-flow constructs which was the reason why it was renamed from LiveScript to JavaScript.

In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.

JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.

Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's class-based model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript programming.

In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.

JavaScript compared to Java
JavaScript Java
Object-oriented. No distinction between types of objects. Inheritance is through the prototype mechanism, and properties and methods can be added to any object dynamically. Class-based. Objects are divided into classes and instances with all inheritance through the class hierarchy. Classes and instances cannot have properties or methods added dynamically.
Variable data types are not declared (dynamic typing). Variable data types must be declared (static typing).
Cannot automatically write to hard disk. Cannot automatically write to hard disk.

For more information on the differences between JavaScript and Java, see the chapter Details of the object model.

JavaScript and the ECMAScript specification

JavaScript is standardized at Ecma International — the European association for standardizing information and communication systems (ECMA was formerly an acronym for the European Computer Manufacturers Association) to deliver a standardized, international programming language based on JavaScript. This standardized version of JavaScript, called ECMAScript, behaves the same way in all applications that support the standard. Companies can use the open standard language to develop their implementation of JavaScript. The ECMAScript standard is documented in the ECMA-262 specification. See New in JavaScript to learn more about different versions of JavaScript and ECMAScript specification editions.

The ECMA-262 standard is also approved by the ISO (International Organization for Standardization) as ISO-16262. You can also find the specification on the Ecma International website. The ECMAScript specification does not describe the Document Object Model (DOM), which is standardized by the World Wide Web Consortium (W3C). The DOM defines the way in which HTML document objects are exposed to your script. To get a better idea about the different technologies that are used when programming with JavaScript, consult the article JavaScript technologies overview.

JavaScript documentation versus the ECMAScript specification

The ECMAScript specification is a set of requirements for implementing ECMAScript; it is useful if you want to implement standards-compliant language features in your ECMAScript implementation or engine (such as SpiderMonkey in Firefox, or v8 in Chrome).

The ECMAScript document is not intended to help script programmers; use the JavaScript documentation for information on writing scripts.

The ECMAScript specification uses terminology and syntax that may be unfamiliar to a JavaScript programmer. Although the description of the language may differ in ECMAScript, the language itself remains the same. JavaScript supports all functionality outlined in the ECMAScript specification.

The JavaScript documentation describes aspects of the language that are appropriate for a JavaScript programmer.

Getting started with JavaScript

Getting started with JavaScript is easy: all you need is a modern Web browser. This guide includes some JavaScript features which are only currently available in the latest versions of Firefox, so using the most recent version of Firefox is recommended.

There are two tools built into Firefox that are useful for experimenting with JavaScript: the Web Console and Scratchpad.

The Web Console

The Web Console shows you information about the currently loaded Web page, and also includes a command line that you can use to execute JavaScript expressions in the current page.

To open the Web Console, select "Web Console" from the "Web Developer" menu, which is under the "Tools" menu in Firefox. It appears at the bottom of the browser window. Along the bottom of the console is a command line that you can use to enter JavaScript, and the output appears in the pane above:

Scratchpad

The Web Console is great for executing single lines of JavaScript, but although you can execute multiple lines, it's not very convenient for that, and you can't save your code samples using the Web Console. So for more complex examples Scratchpad is a better tool.

To open Scratchpad, select "Scratchpad" from the "Web Developer" menu, which is under the "Tools" menu in Firefox. It opens in a separate window and is an editor that you can use to write and execute JavaScript in the browser. You can also save scripts to disk and load them from disk.

If you choose "Inspect", the code in your pad is executed in the browser and the result is inserted back into the pad as a comment:

Hello world

To get started with writing JavaScript, open the Web Console or the Scratchpad and write your first "Hello world" JavaScript code.

function greetMe(user) {
  return "Hi " + user;
}

greetMe("Alice"); // "Hi Alice"

In the following pages, this guide will introduce you to the JavaScript syntax and language features, so that you will be able to write more complex applications.

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Contributors to this page: isomura-bon, teoli, Shuyas
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