TTL cache

Write a class TTLCache (TTL stands for time to live). new TTLCache(defaultTtl) makes an empty cache whose entries live for defaultTtl milliseconds. The tests move time with fake timers, which control both Date.now() and setTimeout.

set(key, value, ttl) stores value under key for ttl ms, or for defaultTtl when ttl is left out. Setting a key that's already there replaces its value and starts its time again. An entry stored at time t is still there at t + ttl - 1 and gone from t + ttl on.

get(key) returns the value, or undefined when the key is missing or has expired. has(key) says whether a live entry exists, even when its value is undefined. delete(key) removes an entry and returns true if a live one was there, false otherwise. size is a getter for the number of live entries.

const cache = new TTLCache(1000);
cache.set("user", { name: "Ada" });
cache.set("token", "abc", 100);
// 100 ms later:
cache.get("token"); // undefined
cache.get("user");  // { name: "Ada" }
cache.size;         // 1

What the tests check

1. 1keeps a value until its ttl has passed

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("a", 1);
   clock.tick(99);
   expect(cache.get("a")).toBe(1);
   expect(cache.has("a")).toBe(true);
   

2. 2forgets a value once its ttl has passed

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("a", 1);
   clock.tick(100);
   expect(cache.get("a")).toBeUndefined();
   expect(cache.has("a")).toBe(false);
   

3. 3a ttl passed to set overrides the default

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("short", 1, 10);
   cache.set("long", 2, 500);
   clock.tick(10);
   expect(cache.get("short")).toBeUndefined();
   clock.tick(400);
   expect(cache.get("long")).toBe(2);
   

4. 4setting a key again replaces the value and restarts its time

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("a", 1);
   clock.tick(60);
   cache.set("a", 2);
   clock.tick(60);
   expect(cache.get("a")).toBe(2);
   clock.tick(40);
   expect(cache.get("a")).toBeUndefined();
   

5. 5has() is true for a live entry whose value is undefined

   useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("nothing", undefined);
   cache.set("zero", 0);
   expect(cache.has("nothing")).toBe(true);
   expect(cache.get("zero")).toBe(0);
   expect(cache.has("missing")).toBe(false);
   

6. 6delete removes an entry and says whether a live one was there

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   cache.set("a", 1);
   cache.set("b", 2, 10);
   expect(cache.delete("a")).toBe(true);
   expect(cache.get("a")).toBeUndefined();
   expect(cache.delete("a")).toBe(false);
   clock.tick(10);
   expect(cache.delete("b")).toBe(false);
   

7. 7size counts only live entries

   const clock = useFakeTimers();
   const cache = new TTLCache(100);
   expect(cache.size).toBe(0);
   cache.set("a", 1, 50);
   cache.set("b", 2);
   expect(cache.size).toBe(2);
   clock.tick(50);
   expect(cache.size).toBe(1);
   clock.tick(50);
   expect(cache.size).toBe(0);
   

Learn more

• Map & SetLesson
• Private fields & methodsLesson