The demonstration aims to present JerryScript [1], a JavaScript engine for Internet of Things (IoT). This is a lightweight JavaScript engine intended to run on a very constrained devices such as microcontrollers: - Only few kilobytes of RAM available to the engine (<64 KB RAM) - Constrained ROM space for the code of the engine (<200 KB ROM) The engine is ECMA-262 5.1 compliant, supports on-device compilation, execution and provides access to peripherals from JavaScript. It powers the IoT.js project [2], which provides interoperable service platform in the world of IoT based on web. This demonstration proves that usage of JavaScript on every constraint device is reasonable and profitable.