UnhandledPromiseRejectionWarning

The treatment of unhandled promise rejections changed fundamentally in Node.js 15 (October 2020). Before Node 15, an unhandled rejection emitted a DeprecationWarning and the process kept running. Starting with Node 15 (and Node 16+ LTS), an unhandled rejection terminates the process with exit code 1 — the same behavior as an uncaught synchronous exception. This behavioral change broke production applications that relied on the lenient old behavior. A common pattern was fire-and-forget async operations: sendAnalyticsEvent(data) called without await or .catch(). Previously harmless, this now crashes the server if sendAnalyticsEvent ever rejects. The unhandledRejection process event (available in all Node versions) is the monitoring hook: process.on('unhandledRejection', (reason) => { logger.error(reason); }). This should be a logging hook, not a rejection suppressor — using it to swallow errors masks bugs. Tools that identify the source: --trace-warnings flag (built into Node) prints the origin stack trace of the unhandled rejection. Setting --unhandled-rejections=warn restores the Node 14 lenient behavior as a temporary migration escape valve but is deprecated and will be removed in a future version. The eslint-plugin-promise package can catch missing .catch() and await patterns statically.