extending.ts

// OUR CODE 
interface Workorder {
    field: string
}

interface WorkorderService {
    create(workorder: Workorder):void
}

// TODO implementation

// Customer code

interface MyWorkorder extends Workorder{
    myfield: string
}

interface MyWorkorderService extends WorkorderService{
    // This creates new method
    //create(workorder: MyWorkorder): void

    // This overrides method
    create(workorder: Workorder): void

}


class MyWorkorderServiceImpl implements MyWorkorderService {
    create(workorder: MyWorkorder){
        workorder.myfield = "test";
    }

    // This overrides method but myfield is not visible.
    // create(workorder: Workorder){
    //     workorder.myfield
    // }
}

// Solution 1 (composition)
class MyWorkorderServiceImpl  {
    lowLevelService: MyWorkorderService;

    constructor(lowLevelService: MyWorkorderService){
        this.lowLevelService = lowLevelService;
    }

    create(workorder: MyWorkorder){
        workorder.myfield = "test";
        this.lowLevelService.create(workorder);
    }

    // This overrides method
    // create(workorder: Workorder){
    //     workorder.myfield
    // }
}

// Solution 3

// Proper abstraction level and no business logic in API. 
// Extending by providing level of abstraction 

// Example

class Step{
    preAction?: function
    postAction?: function
}

const workflow = new Workflow();
workflow.setStepsStrategy(const.LinearStepsStragegy);

Steps[] steps = { SimpleStep, PersistenStep, FormsStep, RemoteDataStep }
steps[0].setMetadata({field:{name: "test"}});
workflow.assignSteps(steps);

// Documentation for each of this interfaces rather than only MODULE interfaces. 
// Aproach similar to https://www.optaplanner.org/