User defined test for is PartiallyRequired?

Question

How can I write a function that checks that SOME fields on a type are required?

The IFinancingModel also has statusDetails which might be undefined on a valid financing so using Required<> is not correct.

export function validFinancing(
    financing: IFinancingModel | undefined,
): financing is Required<IFinancingModel> {
    return !!(
        projectFinancing && 
        projectFinancing.applicationId && 
        projectFinancing.state
    );
}

Answer 1

You can use Pick to select some properties of the type and make them required and make intersect that with the original to produce the desired type:

type RequireSome<T, K extends keyof T> = Required<Pick<T, K>> & T;
interface IFinancingModel {
  applicationId?: number;
  state?: string;
  statusDetails?: string
}

export function validFinancing(
    financing: IFinancingModel | undefined,
): financing is RequireSome<IFinancingModel, 'applicationId' | 'state'> {
    return !!(
        financing && 
        financing.applicationId && 
        financing.state
    );
}
declare let o: IFinancingModel | undefined;
if (validFinancing(o)) {
  o.applicationId.toExponential // number 
  o.state.anchor; // string
  o.statusDetails.big // err can be undefined
}