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Este capítulo apresenta a linguagem JavaScript e discuste alguns dos seus conceitos fundamentais.
O que já deverias saber
Este guia assume que possuís os seguintes conhecimentos:
- Um conhecimento geral sobre Internet e o World Wide Web (WWW).
- Bom conhecimento sobre como desenvolver HyperText Markup Language (HTML).
- Alguma experiência a programar. Caso sejas iniciante a programar, experimenta seguir um dos links para tutoriais que estão presentes na página principal sobre JavaScript.
Onde encontrar informação sobre JavaScript
A documentação de JavaScript no MDN incluí o seguinte:
- Aprender a Web fornece informação para iniciante e introduz conceitos básicos sobre programação e a Internet.
- Guia de JavaScript (este guia) fornece uma visão geral sobre a linguagem JavaScript e os seus objetos.
- Referência de JavaScript fornece conteúdos de referência detalhada para JavaScript.
Se fores iniciante em JavaScript, começa por ler os artigos na área de aprendizagem e o Guia de JavaScript. Assim que compreenderes os fundamentos da linguagem, podes consultar a Referência de JavaScript para obter mais detalhes sobre objetos individuais e declarações.
O que é JavaScript?
JavaScript é uma linguagem de scripting orientada a objetos e que funciona entre plataformas. É uma linguagem pequena e simples. Num ambiente anfitrião (por exemplo, o browser), código JavaScript pode estar ligado a objetos do ambiente e fornecer control programático sobre os mesmos.
O JavaScript contém uma biblioteca padrão de objetos, tais como Array, Date, and Math, e um conjunto fundamental de elementos da linguagem tais como operadores, estruturas de controlo, e statements. Os elementos básicos do JavaScript podem ser extendidos com objetos adicionais para uma variedade de propósitos, por exemplo:
- JavaScript a correr no cliente extende a linguagem básica pois fornece objetos para controlar o browser e o seu Document Object Model (DOM). Por exemplo, extenções de cliente permitem a uma aplicação adicionar elementos num formulário HTML e responder a eventos do utilizador tais como cliques, input adicionado, e navegação na página.
- JavaScript a correr no servidor extende a linguagem básica ao fornecer objetos relevantes para que isso aconteça. Por exemplo, extenções do lado do servidor permitem que uma aplicação comunique com uma base de dados, garante continuidade de informação entre invocações da aplicação, ou executa manipulações de ficheiros no servidor.
JavaScript e Java
JavaScript e Java são linguagens similares em alguns aspetos mas fundamentalmente diferentes noutros. A linguagem JavaScript assemelha-se ao Java mas não tem o static typing nem a validação strong type. O JavaScsript segue a sintaxe de expressões do Java, convenções de nomenclatura e os construtores básicos de controlo de fluxo. Esta última foi a razão pelo qual a linguagem foi renomeada de LiveScript para 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 | 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. | Can automatically write to hard disk. |
For more information on the differences between JavaScript and Java, see the chapter Details of the object model.
JavaScript e a especificação ECMAScript
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) and/or WHATWG (Web Hypertext Application Technology Working Group). 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.
Documentação JavaScript versus especificação ECMAScript
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.
Começar com 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.
A Consola da Web
A Consola da Web 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.
Olá mundo
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.