Elk - a restricted JS engine for embedded systems

Elk is a single-file JavaScript engine for microcontrollers.

Features

• Clean ISO C, ISO C++. Builds on old (VC98) and modern compilers, from 8-bit (e.g. Arduino mini) to 64-bit systems
• No dependencies
• Implements a restricted subset of ES6 with limitations
• Preallocates all necessary memory and never calls malloc, realloc at run time. Upon OOM, the VM is halted
• Object pool, property pool, and string pool sizes are defined at compile time
• The minimal configuration takes only a few hundred bytes of RAM
• RAM usage: an object takes 6 bytes, each property: 16 bytes, a string: length + 6 bytes, any other type: 4 bytes
• Strings are byte strings, not Unicode. For example, 'ы'.length === 2, 'ы'[0] === '\xd1', 'ы'[1] === '\x8b'
• Limitations: max string length is 256 bytes, numbers hold 32-bit float value, no standard JS library
• mJS VM executes JS source directly, no AST/bytecode is generated
• Simple FFI API to inject existing C functions into JS

Embedded example: blinky in JavaScript on Arduino Mini

#define MJS_STRING_POOL_SIZE 200    // Buffer for all strings
#include "elk.c"                    // Sketch -> Add File -> elk.c

extern "C" void myDelay(int x) { delay(x); }
extern "C" void myDigitalWrite(int x, int y) { digitalWrite(x, y); }

void setup() {
  struct vm *vm = js_create();                        // Create JS instance
  js_ffi(vm, "delay", (cfn_t) myDelay, "vi");        	// Import delay()
  js_ffi(vm, "write", (cfn_t) myDigitalWrite, "vii"); // Import write()
  js_eval(vm, "while (1) { write(13, 0); delay(100); write(13, 1); delay(100); }", -1);
}

void loop() { delay(1000); }

Sketch uses 17620 bytes (57%) of program storage space. Maximum is 30720 bytes.
Global variables use 955 bytes (46%) of dynamic memory, leaving 1093 bytes for local variables. Maximum is 2048 bytes.

Supported standard operations and constructs

Name	Operation
Operations	All but !=, ==. Use !==, === instead
typeof	typeof(...)
delete	delete obj.k
while	while (...) {...}
Declarations	let a, b, c = 12.3, d = 'a';
Simple types	let a = null, b = undefined, c = false, d = true;
Functions	let f = function(x, y) { return x + y; };
Objects	let obj = {a: 1, f: function(x) { return x * 2}}; obj.f();

Unsupported standard operations and constructs

Name	Operation
Arrays	let arr = [1, 2, 'hi there']
Loops/switch	for (...) { ... },for (let k in obj) { ... }, do { ... } while (...), switch (...) {...}
Equality	==, != (note: use strict equality ===, !==)
var	var ... (note: use let ...)
Closures	let f = function() { let x = 1; return function() { return x; } };
Const, etc	const ..., await ... , void ... , new ..., instanceof ...
Standard types	No Date, ReGexp, Function, String, Number
Prototypes	No prototype based inheritance

Supported non-standard JS API

Function	Description
s[offset]	Return byte value at offset. s is either a string, or a number. A number is interprepted as uint8_t * pointer. Example: 'abc'[0] returns 0x61. To read a byte at address 0x100, use 0x100[0];.

LICENSE

Dual license: GPLv2 or commercial. For commercial licensing, please contact support@mongoose-os.com