JavaScript is a prototype-based, object-oriented scripting language that is dynamic, weakly typed and has first-class functions.
From the GIS Wiki:
JavaScript is a scripting language used to enable programmatic access to objects within both the client application and other applications. It is primarily used in the form of client-side JavaScript, implemented as an integrated component of the web browser, allowing the development of enhanced user interfaces and dynamic websites. JavaScript is a dialect of the ECMAScript standard and is characterized as a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to look like Java, but to be easier for non-programmers to work with.
JavaScript libraries are often used in web-based GIS solutions for the display of maps. Examples include ESRI's ArcGIS JavaScript API, leaflet.js, and OSGEO's OpenLayers.
