bitcoinjs-lib/ts_src/payments/lazy.ts

export function prop(object: Object, name: string, f: () => any): void {
  Object.defineProperty(object, name, {
    configurable: true,
    enumerable: true,
    get: function() {
      let value = f.call(this);
      this[name] = value;
      return value;
    },
    set: function(value) {
      Object.defineProperty(this, name, {
        configurable: true,
        enumerable: true,
        value: value,
        writable: true,
      });
    },
  });
}

export function value<T>(f: () => T): () => T {
  let value: T;
  return function(): T {
    if (value !== undefined) return value;
    value = f();
    return value;
  };
}