Mongoose OS - Mongoose OS - an IoT firmware development framework
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Mongoose OS is a cross-platform IoT framework that comes with a range of libraries that can be easily used with minimum coding. It is based on the popular Mongoose networking lib for embedded systems. It is design to reduce the development time for product. Supports - ESP32, ESP8266, TICC 3220-3200, STM32 L4-F4-F7.

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good documentation. well supported forum and gitter-chat.
supports both C/C++ & JS.
Pros:rapid development. not a lot of boiler plate. focuses on security aspects as well. multi-platform support.
Cons:open source licence is limited.