Introduction

Acest capitol introduce JavaScript și discută câteva din conceptele sale fundamentale.

Ceea ce ar trebui să știi deja

Acest ghid presupune că deja ai următoarele fundamente:

• O înțelegere generală a Internetului și a World Wide Web (WWW).
• Cunoștințe privind funcționarea HyperText Markup Language (HTML).
• Ceva experiență de programare. Dacă abia te-ai apucat de programare, poți încerca unul din tutorialele de JavaScript legate din prima pagină.

Unde să poți descoperi informații despre JavaScript

Documentația JavaScript din MDN include următoarele:

• Învățarea Web-ului oferă informații începătorilor și introduce concepte de bază din programare și Internet.
• Ghidul JavaScript (acest ghid) oferă o privire generală peste limbajul JavaScript și obiectele sale.
• Referința JavaScript oferă material detaliat de referință pentru JavaScript.

Dacă ești nou în JavaScript, pornește cu articolele din zona de învățare și cu Ghidul JavaScript. De îndată ce ai înțeles bine fundamentele, poți folosi Referința JavaScript pentru a obține mai multe detalii cu privire la obiectele și declarațiile luate separat.

De ce JavaScript?

JavaScript este un limbaj de scriptare multi-platformă, orientat pe obiecte. Este un limbaj ușor și suplu. În interiorul unui mediu gazdă (de exemplu, un browser web), JavaScript poate fi conectat la obiectele mediului pentru a oferi control programatic asupra acestora.

JavaScript conține o colecție standard de obiecte așa cum sunt Array, Date și Math, precum și un set nucleu de elemente de limbaj cum sunt operatorii, structuri de control și declarații. Nucleul JavaScript poate fi extins pentru o varietate de scopuri prin suplimentarea lu prin adăugarea de obiecte, cum ar fi de exemplu:

• JavaScript Client-side extinde limbajul de bază prin adăugarea de obiecte pentru a controla un browser și al său Document Object Model (DOM). De exemplu, extensiile client-side permit unei aplicații să-și plaseze elemente într-un formular HTML și care să răspundă extensions allow an application to place elements on an HTML form and respond la evenimente precum clicuri de mouse, form input-uri și navigarea paginilor.
• JavaScript Server-side extinde nucleul limbajului prin introducerea de obiecte relevante pentru rularea JavaScript pe un server. De exemplu, extensiile la nivel de server permit unei aplicații să comunice cu o bază de date, să ofere continuitatea informației de la o invocare la alta a aplicației sau să manipuleze fișiere pe server.

JavaScript și Java

JavaScript și Java sunt similare în anumite zone dar este fundamental diferit în altele. Limbajul JavaScript seamănă cu Java dar nu oferă concepte precum static typing și verificarea strictă a tipurilor. JavaScript urmează majoritatea sintaxei folosite de Java, convențiile privind numele și constructele de bază privind flow-controlului fiind motivele pentru care a fost redenumit din LiveScript în 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.

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 (Ctrl+Shift+K), select "Web Console" from the "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 (Shift+F4), select "Scratchpad" from the "Developer" menu, which is under the 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.

Hello world

To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:

function greetMe(yourName) {
  alert("Hello " + yourName);
}

greetMe("World");

Select the code in the pad and hit Ctrl+R to watch it unfold in your browser!

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.