The conditional type lacks proper restriction

I am facing an issue with my updateView function that is supposed to merge changes and update a view on the backend. The problem lies in not properly restricting what changes can be passed in based on the type of view. Below is an example of the code snippet:

How do I modify the UpdateView interface to enforce strict restrictions on the allowable changes?

interface BaseView {
    immutableProp?: string;
    mutableProp?: string;
    name: string;
    isCustom: boolean;
}

interface StandardView extends BaseView {
    isCustom: false;
}

interface CustomView extends BaseView {
    isCustom: true;
}

type View = StandardView | CustomView

type StandardMutableFields = Partial<Pick<StandardView, "mutableProp">>
type CustomMutableFields = Partial<Pick<CustomView, "mutableProp" | "name">>


interface UpdateView {
    <T extends View>(viewToChange: T, changes: T extends CustomView ? CustomMutableFields : StandardMutableFields): T
}

const updateView: UpdateView = (viewToChange, changes) => {
    const updatedView = { ...viewToChange, changes };
    // SAVE TO SERVER
    return updatedView;
}

const changeName = (view: View, name: string) => {
    updateView(view, { name: "this is allowed but shouldn't be" }) // should require a check to ensure the view is custom
}

Answer №1

My suggestion for a solution in this scenario (based on the information provided) would be to simplify by removing conditional types and the generic function, and instead utilize overloads.

By using overloads, you eliminate the possibility of calling a function with a union of potential parameter types. This means that the function can only be called with either a CustomView or a StandardView, requiring you to differentiate between the two before making the call:

interface UpdateView {
    (viewToChange: CustomView, changes: CustomMutableFields): CustomView
    (viewToChange: StandardView, changes: StandardMutableFields): StandardView
}

const updateView: UpdateView = (viewToChange: CustomView | StandardView, changes : CustomMutableFields | StandardMutableFields) => {
    const updatedView = { ...viewToChange, changes };
    // SAVE TO SERVER
    return updatedView as any;
}

const changeName = (view: View, name: string) => {
    updateView(view, { name: "this is allowed but shouldn't be" }) // err
    if (view.isCustom) {
        updateView(view, { name: "this is allowed but shouldn't be" }) // ok
    }
}

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