JerryScript is a lightweight JavaScript engine designed to bring the success of JavaScript to small IoT devices like lamps, thermometers, switches and sensors. This class of devices tends to use resource-constrained microcontrollers which are too small to fit a large JavaScript engine like V8 or JavaScriptCore. JerryScript is heavily optimized for low memory consumption and runs on platforms with less than 64KB of RAM and less than 200KB of flash memory. Despite the low footprint, JerryScript is a full-featured JavaScript engine implementing the entire ECMAScript 5.1 standard. It is actively used in production and already runs on more than one million smartwatches! JerryScript is an open source project and has been released under the Apache License 2.0. The talk will include a demo showing JavaScript code executing on top of JerryScript on a resource-constrained microcontroller.