lib/properties.interfaces.ts

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export type PropertyValue = string | boolean | number | undefined;

// names don't matter
interface TypeMap {
    a: string;
    b: boolean;
    c: number;
    d: undefined;
}

export type Widen<T> = T extends T
    ? {
          [P in keyof TypeMap]: T extends TypeMap[P] ? TypeMap[P] : never;
      }[keyof TypeMap]
    : T;

// TODO(jeremy-rifkin): I think the types could use some work here.
// Maybe props<string>(property) should be string | undefined.

function superficialGetter(property: string, defaultValue?: undefined): PropertyValue;
function superficialGetter<T extends PropertyValue>(property: string, defaultValue: Widen<T>): typeof defaultValue;
function superficialGetter<T extends PropertyValue>(property: string, defaultValue?: unknown): T;
function superficialGetter(property: string, defaultValue?: unknown): unknown {
    return; // eslint-disable-line no-useless-return
}

export type PropertyGetter = typeof superficialGetter;