Customize coercion rules for HostObjects with valueOf

[zoryn]

Currently HostObjects that are not primitive types are always coerced to a string in JStoPrimitiveNode.java.
Is there any way to customize this behavior to coerce HostObject into a number? I have a host object which is a number with optional annotations. I want to be able to use it in arithmetic operations in JS code. Ability to specify equivalent of valueOf function for HostObject will be useful in this scenario. This cab be implemented as a new Proxy interface with valueOf method.

[iamstolis]

You are right that ToPrimitive() operation applied to host objects (or other foreign/non-JavaScript objects) should consider valueOf() method. In fact, there is already a pull-request in progress that implements this change. It may take some time before all its details are polished but this feature will be available in next GraalVM release, I believe.

[iamstolis]

There is a workaround that may or may not be applicable in your case. If you wrap your object in a JavaScript Proxy (new Proxy(yourObject, {})) then it should behave as the original object but the valueOf() method will be considered.

[zoryn]

Yes, wrapping with Proxy object will work. Thanks for the quick reply!