mobx-keepalive.ts

import { observable, computed } from "mobx";

class LongLived {
  @observable
  foo = 5;
}

class ShortLived {
  constructor(readonly longLived) {
    this.longLived = longLived;
  }

  get bar() {
    return this.longLived.foo * 2;
  }
}

const longLived = new LongLived();
let sum = 0;
for (let i = 0; i < 100; ++i) {
  const shortLived = new ShortLived(longLived);
  sum += shortLived.bar;
}

console.log(sum);