Define a lazily evaluated property on arbitary objects.

define-lazy-property.ts

type ModifiedObject<
  TObject extends Record<string, unknown>,
  TPropertyName extends string,
  TPropertyValue,
> = TObject & {
  [K in TPropertyName]: TPropertyValue;
};

export const defineLazyProperty = <
  TObject extends Record<string, unknown>,
  TPropertyName extends string,
  TPropertyValue,
>(
  object: TObject,
  propertyName: TPropertyName,
  valueGetter: () => TPropertyValue,
): ModifiedObject<TObject, TPropertyName, TPropertyValue> => {
  const define = (value: TPropertyValue) =>
    Object.defineProperty(object, propertyName, {
      value,
      enumerable: true,
      writable: true,
    });

  Object.defineProperty(object, propertyName, {
    configurable: true,
    enumerable: true,
    get(): TPropertyValue {
      const result = valueGetter();

      define(result);

      return result;
    },
    set(value: TPropertyValue) {
      define(value);
    },
  });

  return object as ModifiedObject<TObject, TPropertyName, TPropertyValue>;
};