This chapter introduces JavaScript and discusses some of its fundamental concepts. What is JavaScript?JavaScript is a cross-platform, object-oriented scripting language. JavaScript is a small, lightweight language; it is not useful as a standalone language, but is designed for easy embedding in other products and applications, such as web browsers. Inside a host environment, JavaScript can be connected to the objects of its environment to provide programmatic control over them. Core JavaScript contains a core set of objects, such as
Through JavaScript's LiveConnect functionality, you can let Java and JavaScript code communicate with each other. From JavaScript, you can instantiate Java objects and access their public methods and fields. From Java, you can access JavaScript objects, properties, and methods. Netscape invented JavaScript, and JavaScript was first used in Netscape browsers. JavaScript and JavaJavaScript 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.
For more information on the differences between JavaScript and Java, see the chapter Details of the Object Model. JavaScript and the ECMAScript SpecificationNetscape invented JavaScript, and JavaScript was first used in Netscape browsers. However, Netscape is working with Ecma International — the European association for standardizing information and communication systems (formerly known as ECMA — the European Computer Manufacturers Association) to deliver a standardized, international programming language based on core 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. The ECMA-262 standard is also approved by the ISO (International Organization for Standardization) as ISO-16262. You can find a PDF version of ECMA-262 (outdated version) at the Mozilla website. 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. Relationship between JavaScript Versions and ECMAScript EditionsNetscape worked closely with Ecma International to produce the ECMAScript Specification (ECMA-262). The following table describes the relationship between JavaScript versions and ECMAScript editions.
Note: ECMA-262, Edition 2 consisted of minor editorial changes and bug fixes to the Edition 1 specification. The current release by the TC39 working group of Ecma International is ECMAScript Edition 5.1
The JavaScript Reference indicates which features of the language are ECMAScript-compliant. JavaScript will always include features that are not part of the ECMAScript Specification; JavaScript is compatible with ECMAScript, while providing additional features. JavaScript Documentation versus the ECMAScript SpecificationThe ECMAScript specification is a set of requirements for implementing ECMAScript; it is useful if you want to determine whether a JavaScript feature is supported in other ECMAScript implementations. If you plan to write JavaScript code that uses only features supported by ECMAScript, then you may need to review the ECMAScript specification. The ECMAScript document is not intended to help script programmers; use the JavaScript documentation for information on writing scripts. JavaScript and ECMAScript TerminologyThe 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. For example:
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