LRU Cache

class LruCache {
    constructor(capacity) {
        if (!Number.isInteger(capacity) || capacity < 1) {
            throw new RangeError('Capacity must be a positive integer.');
        }

        this.capacity = capacity;
        this.items = new Map();
    }

    get size() {
        return this.items.size;
    }

    has(key) {
        return this.peek(key) !== undefined;
    }

    get(key) {
        const entry = this.read(key);
        if (!entry) {
            return undefined;
        }

        this.items.delete(key);
        this.items.set(key, entry);
        return entry.value;
    }

    peek(key) {
        const entry = this.read(key);
        return entry ? entry.value : undefined;
    }

    set(key, value, ttlMs = null) {
        if (value === undefined) {
            throw new TypeError('undefined cannot be stored. Use delete() instead.');
        }

        if (this.items.has(key)) {
            this.items.delete(key);
        } else if (this.items.size >= this.capacity) {
            this.dropExpired();
            if (this.items.size >= this.capacity) {
                const oldest = this.items.keys().next().value;
                this.items.delete(oldest);
            }
        }

        this.items.set(key, {
            value,
            expiresAt: ttlMs == null ? null : Date.now() + ttlMs,
        });

        return this;
    }

    delete(key) {
        return this.items.delete(key);
    }

    clear() {
        this.items.clear();
    }

    read(key) {
        const entry = this.items.get(key);
        if (!entry) {
            return null;
        }

        if (entry.expiresAt != null && Date.now() > entry.expiresAt) {
            this.items.delete(key);
            return null;
        }

        return entry;
    }

    dropExpired() {
        for (const [key, entry] of this.items) {
            if (entry.expiresAt != null && Date.now() > entry.expiresAt) {
                this.items.delete(key);
            }
        }
    }
}

const cache = new LruCache(3);

cache.set('a', 1).set('b', 2).set('c', 3);
cache.get('a');
cache.set('d', 4); // drops b

console.log(cache.get('b')); // undefined
console.log(cache.peek('c')); // 3, order unchanged
console.log([...cache.items.keys()]); // ['c', 'a', 'd']

cache.set('token', 'abc', 2000); // expires in 2s