defaultObject.spec.ts

import { defaultObject } from './defaultObject';

describe('defaultObject', () => {
  it('should assign default values based on the provided factory', () => {
    const output = defaultObject(() => [] as string[]);
    output.id.push('3');

    expect(cleanupProxy(output)).toEqual({ id: ['3'] });
  });

  it('should handle nested calls "defaultObject"', () => {
    const output = defaultObject(() => defaultObject(() => 4));
    output.layer1.layer2 += 3;

    expect(cleanupProxy(output)).toEqual({ layer1: { layer2: 7 } });
  });
});

export function cleanupProxy<T extends Object>(object: T): T {
  return Object.entries({ ...object }).reduce((output, [key, value]) => {
    const shouldClean = typeof value === 'object' && !Array.isArray(value);
    return { ...output, [key]: shouldClean ? cleanupProxy(value) : value };
  }, {} as T);
}

defaultObject.ts

export function defaultObject<T extends any>(
  factory: () => T
): { [key: string]: T } {
  return new Proxy(
    {},
    {
      get(target: { [key: string]: T }, key: string) {
        if (!(key in target)) {
          target[key] = factory();
        }
        return target[key];
      },
    }
  );
}