IoT.js is a platform for IoT applications written in JavaScript. With IoT.js developers can create IoT services that communicate with each other and the outside world. IoT.js is a lightweight version of Node.js and designed to bring the success of Node.js 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 full Node.js stack. In order to meet those constraints, IoT.js runs on top of JerryScript which is a lightweight JavaScript engine running 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. Both IoT.js and JerryScript are open source projects released under the Apache License. The talk will include a demo running a Node.js module with IoT.js on a resource-constrained microcontroller.