Duktape (http://duktape.org/) – A small footprint, easily embeddable Ecmascript E5/E5.1 engine.
Espruino (http://www.espruino.com/) – A very small footprint interpreter specifically for microcontrollers. Can run in less than 8 kB of RAM by executing from source (rather than bytecode).
JerryScript (https://github.com/jerryscript-project/jerryscript) – A very small interpreter developed by Samsung for IoT application.
Kameleon (https://kameleon.io) – This is a fork of JerryScript with a nice web IDE.
MuJS (https://mujs.com/) – A lightweight ECMAScript interpreter library, designed for embedding in other software to extend them with scripting capabilities. Originally developed for MuPDF.
QuickJS (https://bellard.org/quickjs/) – a small JS VM developed by Fabrice Bellard from QEMU fame.
Throwback (https://github.com/getfilament/throwback) – The smallest JS engine for microcontroller, still in early stage of development
So, we have at least 12 JS engine options to use with microcontrollers. From this listing I think JerryScript is a step ahead, but Kameleon is evolving to be a real alternative for embedded systems.