Question 1

mJS vs Duktape for embedded JavaScript?

Accepted Answer

mJS is lighter (50KB flash vs. Duktape's ~200KB) and focuses on direct C integration with FFI, but Duktape offers more ES5/6 features and a richer standard library. Choose mJS for extreme resource constraints, Duktape for more feature-complete scripting on slightly larger systems.

Question 2

How to call a C function with structs in mJS?

Accepted Answer

Use the s2o helper with struct descriptors: define C struct members in an mjs_c_struct_member array, get a pointer via FFI, then call s2o(ptr, descr) in JS to convert to an object. For complex cases, write a C wrapper function to simplify the interface.

Question 3

What JavaScript features are missing in mJS?

Accepted Answer

mJS lacks closures, exceptions, Unicode strings, standard library objects (String, Date, etc.), var, == operator, for..of, arrow functions, promises, classes, and many ES6+ syntax features. It supports only lexical scoping, let, ===, and basic JSON/array methods.

Question 4

Can mJS run on ESP32?

Accepted Answer

Yes, mJS is part of MongooseOS and commonly used on ESP32 and other microcontrollers. Its small footprint (50KB flash) fits within ESP32's memory constraints, and it can interface with C libraries for IoT tasks like GPIO control or networking.

Question 5

Is mJS suitable for real-time systems?

Accepted Answer

Potentially, but with caveats: its minimal GC and deterministic execution can help, but lack of exceptions and asynchronous features might limit real-time error handling. Best for simple scripting in hard real-time loops where predictable performance is critical.

Question 6

How does mJS handle memory management?

Accepted Answer

mJS includes a garbage collector (gc function) that can reclaim RAM, with an option for full collection to return memory to the OS. However, memory is limited (<1KB RAM usage), so developers must carefully manage allocations and avoid memory leaks in embedded code.

mJS

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