---
title: "Moddable JS Engine"
canonical_url: https://developer.repebble.com/pebbleos/docs/third_party/moddable/
last_updated: 2026-10-21
sdk_version: 4.33.1
---
# Moddable JS Engine

Source: https://developer.repebble.com/pebbleos/docs/third_party/moddable/

PebbleOS supports the [Moddable SDK](https://github.com/coredevices/moddable)’s XS JavaScript engine. The Moddable integration provides a lightweight sandbox for running JavaScript applications and watchfaces on Pebble hardware.

## Building with Moddable

Moddable is enabled by default on platforms whose board defconfig sets `CONFIG_MODDABLE_XS=y`. You can override this at configure time by passing `-DCONFIG_MODDABLE_XS=y` or `-DCONFIG_MODDABLE_XS=n` to `pbl configure`. PRF (recovery) builds always disable the engine.

## Installable applications

The JavaScript is stored in a resource. A small native C application bootstraps JavaScript execution by calling `moddable_createMachine`.

The JavaScript itself is a Moddable SDK **mod** , which is a precompiled collection of modules. The modules are compiled using the `mcrun` tool in the Moddable SDK. See Moddable’s [Pebble Examples repository](https://github.com/moddable-OpenSource/pebble-examples) for further details. The Pebble resource containing the precompiled mod is currently loaded into RAM for execution; eventually, it should be executed directly from flash (this is what the Moddable SDK does on most platforms).

## Sandbox

The JavaScript in installable applications runs inside a lightweight sandbox, which can limit access to certain JavaScript global variables and modules.

Installable applications can be either a normal application or a watchface. The sandbox prevents watchfaces from subscribing to the Pebble hardware buttons (which is slightly redundant since PebbleOS also blocks them — but PebbleOS blocks them silently, whereas the sandbox throws an exception, which is more clear to developers).

Generated from PebbleOS [3a7b71785](https://github.com/coredevices/pebbleos/commit/3a7b71785fd439666c4a8bc116f561140e8dec01). This page is maintained in the pebbleos repository: [docs/third\_party/moddable.md](https://github.com/coredevices/pebbleos/blob/3a7b71785fd439666c4a8bc116f561140e8dec01/docs/third_party/moddable.md).