GraalJS support

It is possible to use the GraalJS runtime instead of Rhino for JavaScript evaluation. GraalJS is fully ECMAScript 2022 compliant while Rhino only supports ECMAScript 5.

Right now, Rhino is the default JavaScript engine, but we intend to make GraalJS the default soon and then deprecate and remove Rhino.

This issue track the progress.

Enable GraalJS via flow config

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

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

Enable GraalJS via environment variable

The env var will have no effect when running on Maestro Cloud. Use the flow config above 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.

TL;DR is that the variable scoping when using GraalJS 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

Case	Code sample	Rhino JS	GraalJS
Redeclaring variables across scripts	appId: com.example.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.example --- - evalScript: ${const name = 'joe'} - evalScript: ${name === 'joe'}	✅ Variables are accessible across scripts	❌ Variables can't be accessed across scripts
Handling special characters	appId: com.example.app env: FOO: \ --- - tapOn: Username - inputText: ${FOO}	❌ Single backslash causes an exception	✅ Single backslash and all other special chars are handled correctly