Express middleware to log HTTP requests on Firebase functions

logger.middleware.ts

import { NextFunction, Request, Response } from 'express';
import { LogEntry, LogSeverity, write } from 'firebase-functions/lib/logger';

export function functionRequestLogger(req: Request, res: Response, next: NextFunction) {
    const start = Date.now();
    res.on('finish', () => {
        const code = res.statusCode;
        const logEntry: LogEntry = {
            severity: getSeverity(code),
            httpRequest: {
                requestMethod: req.method,
                status: code,
                requestUrl: req.url,
                remoteIp: req.ip,
                referer: req.get('Referrer'),
                userAgent: req.get('User-Agent'),
                latency: latency(start)
            },
        };
        write(logEntry);
    });
    next();
}

function latency(startMs: number) {
    const diff = Date.now() - startMs;
    const seconds = Math.floor(diff / 1000);
    const remainingMs = diff - seconds * 1000;
    return {
        seconds: seconds,
        nanos: remainingMs * 1000000
    };
}

function getSeverity(code: number): LogSeverity {
    if (code >= 100 && code < 400) {
        return 'INFO';
    } else if (code >= 500) {
        return 'CRITICAL';
    } else if ([400, 401, 404].includes(code)) {
        return 'WARNING';
    }
    return 'ERROR';
}