memo.ts

export function memo<T extends (...args: any[]) => any>(fn: T): T {
  const call = Object.assign(fn, { cache: null as ReturnType<T> | null })

  return ((...args: Parameters<T>) => call.cache ??= call(...args)) as T
}

export class Foo {
  bar = memo(() => 1)
}

overwrite.ts

export type Mutable<T> = {
  -readonly [K in keyof T]: T[K]
}

export class Foo {
  get bar() {
    return cache(this, 'bar', () => 1)
  }
}

/**
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get#smart_self-overwriting_lazy_getters
 */
function cache<T, K extends keyof T, V extends T[K]>(self: T, prop: K, value: () => V) {
  delete self[prop]
  return self[prop] = value()
}