Level 5: Async and functions · Exercise 74 of 76 · Free

LRU cache

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Return a cache object that holds at most `capacity` entries (`capacity` is at least 1) with three methods. `get(key)` returns the stored value, or `undefined` when the key is missing, and marks the key as the most recently used. `set(key, value)` stores or replaces the value and marks the key as the most recently used; when the cache then holds more than `capacity` entries, the least recently used key is removed. `keys()` returns an array of the keys from least to most recently used. Values such as `0`, `false` and `null` are stored and returned like any other, and `1` and `"1"` are different keys.

What this exercise teaches

Deleting and re-inserting moves a key to the end of a Map, which turns the Map's insertion order into recency order for free. Checking has() keeps falsy values working, and get must refresh too, otherwise a key that is read often is still the first to go.

A hint if you get stuck

A `Map` remembers insertion order. To mark a key as most recent, delete it and set it again; the least recently used key is then `map.keys().next().value`. Use `map.has(key)` rather than testing the value.