JavaScript is a high-level, dynamic, untyped, and interpreted programming language.

It has been standardized in the ECMAScript language specification.

Alongside HTML and CSS, JavaScript is one of the three core technologies of World Wide Web content production; the majority of websites employ it, and all modern Web browsers support it without the need for plug-ins.

JavaScript is prototype-based with first-class functions, making it a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.

It has an API for working with text, arrays, dates and regular expressions, but does not include any I/O, such as networking, storage, or graphics facilities, relying for these upon the host environment in which it is embedded.

Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods.

A feature of objects is that an object's procedures can access and often modify the data fields of the object with which they are associated (objects have a notion of "this" or "self").

In OOP, computer programs are designed by making them out of objects that interact with one another.

There is significant diversity of OOP languages, but the most popular ones are class-based, meaning that objects are instances of classes, which typically also determine their type.

A web page (webpage or Web page) is a document that is suitable for the World Wide Web and web browsers.

A web browser displays a web page on a monitor or mobile device.

The web page is what displays, but the term also refers to a computer file, usually written in HTML or comparable markup language.

Web browsers coordinate the various web resource elements for the written web page, such as style sheets, scripts, and images, to present the web page.

In programming languages, closures (also lexical closures or function closures) are techniques for implementing lexically scoped name binding in languages with first-class functions.

Operationally, a closure is a record storing a function together with an environment: a mapping associating each free variable of the function (variables that are used locally, but defined in an enclosing scope) with the value or reference to which the name was bound when the closure was created.

A closure—unlike a plain function—allows the function to access those captured variables through the closure's copies of their values or references, even when the function is invoked outside their scope.

In computing, an event is an action or occurrence recognized by software that may be handled by the software.

Computer events can be generated or triggered by the system, by the user or in other ways.

Typically, events are handled synchronously with the program flow, that is, the software may have one or more dedicated places where events are handled, frequently an event loop.

A source of events includes the user, who may interact with the software by way of, for example, keystrokes on the keyboard.

Another source is a hardware device such as a timer.

Software can also trigger its own set of events into the event loop, e.g. to communicate the completion of a task.

Software that changes its behavior in response to events is said to be event-driven, often with the goal of being interactive.