node.js event loop delay monitor (measures max CPU time spent processing ticks)

process-monitor.js

const NS_PER_SEC = 1e9;
const NS_PER_MS = 1e6;
const SAMPLE_INTERVAL = 10;

class ProcessMonitor {
    start(interval) {
        let prevTime = [NaN, NaN];

        this.maxEventLoopDelay = 0;
        if (!interval) {
            interval = 5 * 1000;
        }

        this.samplerInterval = setInterval(() => {
            const now = process.hrtime();
            const diff = [now[0] - prevTime[0], now[1] - prevTime[1]];

            if (!isNaN(diff[0])) {
                const delay = ((diff[0] * NS_PER_SEC + diff[1]) / NS_PER_MS) - SAMPLE_INTERVAL;

                if (delay > this.maxEventLoopDelay) {
                    this.maxEventLoopDelay = delay;
                }
            }

            prevTime = now;
        }, SAMPLE_INTERVAL);

        this.monitorInterval = setInterval(() => {
            // Feel free to add this to your stats backend
            console.log(this.maxEventLoopDelay + " ms");
            this.maxEventLoopDelay = 0;
        }, interval);
    }

    stop() {
        clearTimeout(this.samplerInterval);
        clearTimeout(this.monitorInterval);
    }
}

module.exports = ProcessMonitor;

/* 
This is fast, simple and no external dependencies
Usage:
        const monitor = new ProcessMonitor();
        monitor.start();
Outputs:
[2017-12-22 14:52:52] [LOG]   [pid:10396] 16.39679 ms
[2017-12-22 14:52:57] [LOG]   [pid:10396] 47.719075 ms
[2017-12-22 14:53:03] [LOG]   [pid:10396] 1062.336003 ms <--- I simulate CPU load here
[2017-12-22 14:53:08] [LOG]   [pid:10396] 823.406753 ms
[2017-12-22 14:53:13] [LOG]   [pid:10396] 10.119005999999999 ms
*/

Inspired by https://www.npmjs.com/package/heavy
and https://www.npmjs.com/package/event-loop-monitor