GraalJS support

It is possible to use GraalJS instead of Rhino JS for javascript evaluation. GraalJS is fully ECMAScript 2022 compliant while Rhino JS only supports ES5.

This feature is experimental for now, but we intend to make GraalJS the default if all goes well.

Enable GraalJS via Flow config

ℹ️ Note This only has an effect if present at the top-level flow. It is ignored when included in any subflows.

appId: com.example.app
jsEngine: graaljs
---
# Note: The ?? operator is an example of an ES2020 feature and is not supported by Rhino JS
- inputText: ${null ?? 'foo'}

Enable GraalJS via environment variable

⚠️ Warning This env var will have no effect when running on Maestro Cloud. Use the Flow config above instead to opt into GraalJS on Maestro Cloud.

export MAESTRO_USE_GRAALJS=true
maestro test my-flow.yaml

GraalJS behavior differences

There are some differences between the new GraalJsEngine and the current RhinoJsEngine implementation that are worth noting. All of the differences below and some others are documented and tested by the GraalJsEngineTest and RhinoJsEngineTest tests.

tldr; The variable scoping with the GraalJS implementation is more consistent and understandable

Rhino JS	GraalJS
Can not reuse variable names across scripts within the same Flow Can reuse variable names if redeclaration lives in a subflow Can access variables declared earlier in the Flow in a separate script output variable is shared across all scripts	Variables are local to each script output variable is shared across all scripts

Some examples

	Example	Rhino JS	GraalJS
Redeclaring variables across scripts	appId: com.example --- - evalScript: ${const foo = null} - evalScript: ${const foo = null}	❌ Variable redeclarations throw an error	✅ Variable names can be reused across scripts
Accessing variables across scripts	appId: com.example --- - evalScript: ${const name = 'joe'} - assertTrue: ${name === 'joe'}	✅ Variables are accessible across scripts	❌ Variables can't be accessed across scripts
Special character handling	appId: com.example.app env: FOO: \ --- - inputText: ${FOO}	❌ Single backslash causes an exception	✅ Single backslash and all other special chars are handled correctly