LRUCache.js

/*
LRU Cache
Stores a limited number of items.- capacity. 

When it reaches capacity and a new item is added, it evicts the least recently used item.

Accessing (reading/writing) an item makes it the most recently used.

JavaScript Map
O(1) read/write/delete by key
Insertion order tracking — first inserted = least recently used
*/

class LRUCache {
    constructor(capacity = 2) {
        this.capacity = capacity;
        this.cache = new Map();
    }

    put(key, value){
        if(this.cache.size === this.capacity) {
            // Evict the least recently used item (first key)
            const firstKey = this.cache.keys().next().value;  
            this.cache.delete(firstKey)    
        }
        
        if(this.cache.has(key)){
            this.cache.delete(key)
        }

        this.cache.set(key, value);
    }

    get(key) {
        if(!this.cache.has(key)) return -1;

        // if the item exists, move it to the top.

        let value = this.cache.get(key);
        this.cache.delete(key);
        this.cache.set(key, value)

        return value;
    }
}



const LRU = new LRUCache(2);
LRU.put(1, 1); //{1 => 1}
LRU.put(2, 2); // {1 => 1, 2=> 2}
console.log(LRU.get(1)); // returns 1
console.log(LRU.get(2)); // returns 2
LRU.put(3, 3); // removes key 1 because it's least recently used { 2=>2 , 3 => 3}
console.log(LRU.get(3)); // returns 3 
console.log(LRU.get(1)); // returns -1 (not found)