What Is JavaScript?

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JavaScript is the Microsoft implementation of the ECMAScript 5th Edition (ES5) language specification. This overview is intended to help you get started with JavaScript.

JavaScript is an interpreted, object-based scripting language. Although it has fewer capabilities than full-fledged object-oriented languages like C++, JavaScript is more than sufficiently powerful for its intended purposes.

JavaScript is not a cut-down version of another language (it is only distantly and indirectly related to Java, for example), nor is it a simplification of anything. It is, however, limited. You cannot write stand-alone applications in it, for example, and it has no built-in support for reading or writing files. Moreover, JavaScript scripts can run only in the presence of an interpreter or "host", such as Internet Explorer.

JavaScript is a loosely typed language. Loosely typed means you do not have to declare the data types of variables explicitly. In fact, JavaScript takes it one step further. You cannot explicitly declare data types in JavaScript. Moreover, in many cases JavaScript performs conversions automatically when needed. For instance, if you add a number to an item consisting of text (a string), the number is converted to text.

The rest of this user's guide is an overview of JavaScript features. For full details of the language implementation, consult the language reference.