Synchronous logger lib (Winston) can't use headers() function anymore now it's async

[clemp6r]

Hi,

I use the Winston logging library for my Next.js app.
I have a custom format used to display some info found in request headers (especially the request ID provided by Heroku) in all log entries.

However, now that the headers() method is asynchronous, I can't do that anymore. Indeed, Winston formatters can't be asynchronous.

I know I could use patch-package to globally expose the request, but this is not a long term solution.

Any idea how I could work around this?

See below the code I currently use with the as unknown as UnsafeUnwrappedHeaders temporary workaround.

import { headers, type UnsafeUnwrappedHeaders } from "next/headers";
import { createLogger, format, transports } from "winston";

const { colorize, combine, printf } = format;

const myFormat = printf((info) => {
  let requestId;
  try {
    const currentHeaders = headers() as unknown as UnsafeUnwrappedHeaders;
    requestId = currentHeaders.get("x-request-id");
  } catch (e) {
    // not in a request context
  }

  const logParts: string[] = [];
  logParts.push(`[${info.level}]`);

  if (requestId) {
    logParts.push(`[${requestId}]`);
  }

  logParts.push(info.message);

  (...)

  return logParts.join(" ");
});

const logger = createLogger({
  level: "debug",
  format: combine(fixErrors(), colorize(), myFormat),
  transports: [new transports.Console()],
});

export default logger;

[clemp6r]

I may have found a solution by wrapping the logger:

export default {
  debug(message: string, ...meta: any[]) {
    logWithHeaders("debug", message, ...meta);
  },
  info(message: string, ...meta: any[]) {
    logWithHeaders("info", message, ...meta);
  },
  warn(message: string, ...meta: any[]) {
    logWithHeaders("warn", message, ...meta);
  },
  error(message: string, ...meta: any[]) {
    logWithHeaders("error", message, ...meta);
  },
};

function logWithHeaders(level: string, message: string, ...meta: any[]) {
  let headersPromise: Promise<Headers | undefined>;
  try {
    headersPromise = headers().catch(() => undefined);
  } catch (e) {
    headersPromise = Promise.resolve(undefined);
  }

  headersPromise.then((headerList) =>
    logger.child({ headerList }).log(level, message, ...meta),
  );
}

Now I have access to the headers in the info object.