I´m traying to implement the js library Handsonetable and Hyperformula (leatests versions) with a application written in java with GWT.

I´m using JsInterop to integrate but there are parts that i don´t know how to map in java.
As example:

export interface ImplementedFunctions {
  [formulaId: string]: FunctionMetadata,
}

That is used to implement a custom function to hyperformula, the providen example is:

export class MyCustomPlugin extends FunctionPlugin {}


MyCustomPlugin.implementedFunctions = {
  DOUBLE_RANGE: {
    method: 'doubleRange',
    arraySizeMethod: 'doubleRangeResultArraySize',
    parameters: [{ argumentType: FunctionArgumentType.RANGE }],
  },
};

As you can see uses typescript index signature.

How can i implement that code in JsInterop.

Any help is appreciated.