A JavaScript Virtual Machine based on JerryScript.


This library allows running JavaScript in a sandbox on all architectures supported by Sming. The library uses JerryScript as JavaScript Virtual Machine (VM).


The diagram above shows the interactions between the major components of JerryScript: Parser and VM. Parser performs translation of input ECMAScript application into the byte-code. Prepared bytecode is executed by the Virtual Machine that performs interpretation. Source: Official JerryScript site.

To save space and be able to run JerryScript on an embedded device Sming compiles this library without a parser. This means that the JavaScript files have to be compiled before landing on the device. See the samples below to learn more.

Console messages

Two external functions are made available for scripts to use:

  • print for normal console output (shown in AQUA)

  • alert for messages in debug mode (show in RED)

VM integrity

The Jerryscript VM may encounter situations where it cannot continue execution, such as memory allocation failure or invalid bytecode.

Fatal errors cause engine execution to halt. It must be “re-booted” (in a virtual sense). This requires application co-operation as scripts may need to be re-loaded, external functions re-registered, etc.

If intending to run unverified javascript it is especially important to guard all calls into Jerryscript using the library exception handler:

   // Call into jerryscript engine
   // Engine has failed

Place code here which sets up jerryscript values, etc. then calls into Jerryscript. Data is allocated on local stack or in jerryscript engine.

DO NOT perform any system heap allocation here, such as creating or modifying String) objects. If required, instantiate those above.


Jerryscript ust be re-initialised before any further calls.

  • De-allocate any globally held jerryscript values

  • Call JS::cleanup()

  • Call JS::init() and any other necessary initialisation

It is recommended to provide a single function for both initial jerryscript initialisation and re-initialisation.

See the Event_Jsvm sample for a demonstratation.

Note that the VM uses own static heap (sized by JERRY_GLOBAL_HEAP_SIZE) so main system heap is unaffected.

When JERRY_ENABLE_DEBUG is enabled it may not be possible to recover because VM objects may be left with invalid reference counts, for example, which will cause Jerryscript::cleanup() to fail. Applications should generally be built with this setting disabled (the default).


Sming is a single-threaded framework so requires co-operation to perform multi-tasking. This means javascript calls should be brief so as not to disrupt the system.

If a javascript call takes too long then the system watchdog will timeout and reset the system. This is generally not helpful, so a separate watchdog is implemented for the Jerryscript virtual machine.

By default, the watchdog is disabled. To enable it, add a call during initialisation:

// Enable the watchdog with a 100ms timeout (0 to disable it)

Now, if a call takes more than 100ms then a fatal error will be thrown as discussed above.

The setting can be changed at any time and is unaffected by jerryscript engine resets.

External Contexts

By default, a single, global VM context is created in which all code is loaded and executed. If multiple javascript applications are running then if one fails it will take out all the others.

Enabling the JERRY_EXTERNAL_CONTEXT setting allows use of the JS::Context class to run multiple isolated instances. See the Basic Context example for details.

Configuration variables


default: 0 (disabled)

Enable assertions and debug messages in jerryscript library. Should be left disabled unless debugging the jerryscript engine itself.


default: 1 (enabled)

Enable jerryscript heap memory tracking.


default: 0 (disabled)

Enable text messages for common errors. Consumes ~2K RAM on ESP8266.


default: 1 (enabled)

Compact (minimal profile) compilation profile makes the JerryScript library smaller.

Set to 0 to use profile.


default: 0 (disabled)

Enable to build library with javascript parser enabled. Required for use of Jerryscript::eval() function.


default: 0 (disabled)

Enable this setting to make use of JS::Context to run multiple isolated instances.


default: 1 (in KB)

Size of the jerryscript global heap in kilobytes. Increase as required.


Read-only. Path to the snapshot compiler for use by applications.


default: undefined

Snap files can be created during the build stage by setting this variable in your project’s file:


All .js files will be compiled into .snap files and written to APP_JS_SNAPDIR.


default: out/jerryscript

Location to write generated .snap files.


default: undefined

Set this if action required when snaps are updated, for example to rebuild SPIFFS image or update application.



Location of pre-compiled web compiler. See Advanced-Jsvm sample for example usage.


The initial work on the JerryScript library for Sming was done as part of the U:Kit project.


Used by

Environment Variables

SoC support

  • esp32

  • esp32c2

  • esp32c3

  • esp32s2

  • esp32s3

  • esp8266

  • host

  • rp2040

Submodule: `jerryscript <>`__