How do you implement a custom event loop with priority queues?

Question

Accepted Answer

**Priority event loop** — process events by priority.

```javascript
class PriorityQueue {
    constructor() {
        this.items = [];
    }

enqueue(item, priority) {
        this.items.push({ item, priority });
        this.items.sort((a, b) => a.priority - b.priority);
    }

dequeue() {
        return this.items.shift()?.item;
    }

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

class PriorityEventLoop {
    constructor() {
        this.queue = new PriorityQueue();
        this.running = false;
    }

enqueue(fn, priority = 5) {
        this.queue.enqueue(fn, priority);
        if (!this.running) this.run();
    }

run() {
        this.running = true;

const processNext = () => {
            if (this.queue.size === 0) {
                this.running = false;
                return;
            }

const fn = this.queue.dequeue();
            try {
                fn();
            } catch (err) {
                console.error("Error:", err);
            }

process.nextTick(processNext);
        };

process.nextTick(processNext);
    }
}

// Usage
const loop = new PriorityEventLoop();

loop.enqueue(() => console.log("Low priority"), 10);
loop.enqueue(() => console.log("High priority"), 1);
loop.enqueue(() => console.log("Medium priority"), 5);

// Output: High, Medium, Low
```

Node.js: How do you implement a custom event loop with priority queues?