[Callable Constructors in TypeScript] How to Merge Ambient Class Declaration with Function Declaration #typescript #tips

deep-boolean.ts

// requires TypeScript v3.6+

export { DeepBoolean };

type Mutable<T> = {
  -readonly [P in keyof T]: T[P];
};

declare class DeepBoolean<T = any> {
  constructor(source: T);
  readonly source: T;
  toString(): string;
  valueOf(): boolean;
}

function DeepBoolean(this: undefined | void, obj: any): boolean;
function DeepBoolean(this: DeepBoolean, obj: any): DeepBoolean;
function DeepBoolean(
  this: DeepBoolean | undefined | void,
  obj: any,
): DeepBoolean | boolean {
  if (!(this instanceof DeepBoolean)) return checkDeepTruth(obj);

  (this as Mutable<DeepBoolean>).source = obj;
  this.valueOf = () => checkDeepTruth(obj);
  this.toString = () => String(checkDeepTruth(obj));

  return this;
}

function checkDeepTruth(obj: any): boolean {
  return !!obj && (
    typeof obj === "object"
      ? Object.keys(obj).every(key => checkDeepTruth(obj[key]))
      : true
  );
}

// EXAMPLE
const source = [{ a: { b: true, n: 1, s: 'TypeScript' } }];
const dB = new DeepBoolean(source);

console.log(dB.valueOf());         // true
console.log(DeepBoolean(source));  // true

source[0].a.s = '';

console.log(dB.valueOf());         // false
console.log(DeepBoolean(source));  // false