Learn TypeScript 3 by Building Web Applications

Implementing getChartFormDetails and using the property shorthand notation

This method will be used by the controller to retrieve the form selections.

Add the definition of the method as follows:

getChartFormDetails(): { error?: string; countryId?: string; indicator?: string; fromYear?: number; toYear?: number, chartType?: string } { 
    // Continue here
}  

As stated before, notice the return type again. We reuse the same trick as in the previous chapter, by defining a custom type including an optional error. Then, we check the different form fields and report back to the user when something is invalid.

Note that you could easily abstract the custom response type using a generic type such as type Result<DataType, ErrorType> = { error?: ErrorType; ...