Mjs

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mJS is an embedded JavaScript engine for C/C++ that is designed for microcontrollers with limited resources. It implements a strict subset of ES6, focusing on a small footprint and simple C/C++ interoperability.

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At a glance

Pricing

Open Source

Free tier

Yes

API

Yes

Skill level

Technical

About

What is mjs?

mJS is an embedded JavaScript engine specifically engineered for microcontrollers and other resource-constrained environments. It implements a strict subset of ES6 (JavaScript version 6), ensuring that any valid mJS code is also valid ES6 code, though not vice-versa due to its intentional restrictions. Key design goals include a small memory footprint, requiring approximately 50KB of flash and less than 1KB of RAM on 32-bit ARM systems. It offers seamless C/C++ interoperability through a Foreign Function Interface (FFI), allowing direct calls to existing C functions and supporting callbacks. The engine is part of MongooseOS, enabling scripting for IoT devices. It provides built-in APIs for JSON parsing, string manipulation, and memory management, along with a helper for converting C structs to JS objects.

Best used for

Ideal for developers who need to add scripting capabilities to embedded systems, integrate C/C++ code with JavaScript, and develop IoT devices with limited resources. Especially valuable for creating efficient and flexible firmware where a small footprint and direct hardware interaction are critical.

Common actions

embed JavaScript

script microcontrollers

integrate C/C++

develop IoT devices

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Capabilities

Key features

Embedded JavaScript engine
C/C++ interoperability
Small memory footprint
ES6 subset implementation
Foreign Function Interface
JSON parsing/stringifying

Target Audience

developer

Integrations

Not yet documented

Pricing & Plans

Open Source

Free

FAQs

What are the main restrictions of mJS compared to full ES6?

mJS implements a strict subset of ES6, omitting features like standard library objects (String, Number, RegExp), closures (only lexical scoping), exceptions, 'new' operator, 'var', 'for..of', arrow functions, destructors, generators, proxies, promises, getters/setters, valueOf, prototypes, classes, template strings, '==' or '!=' (only '===' and '!==').

How does mJS handle C/C++ interoperability?

mJS uses a Foreign Function Interface (FFI) that allows direct calls to existing C functions with arbitrary signatures. It supports simple argument types like int, double, char*, and void*. It also provides mechanisms for callbacks and converting C structs to JavaScript objects without glue code.

What kind of memory footprint does mJS have?

mJS is designed for microcontrollers with limited resources. On a 32-bit ARM system, the engine typically requires about 50KB of flash memory and less than 1KB of RAM, making it suitable for highly constrained embedded environments.

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