Node.js Command Line Console Log Level

Question

How do you set the log level for node when starting it from the command line? I admit, i'm a node.js newbie, but looking for something like node myapp.js --loglevel warn

Node provides no intrinsic logging, other than printing an error message on unhandled exceptions, so there is no loglevel to set. — Rob Raisch
– Rob Raisch, Commented Sep 22, 2012 at 4:29
To add to what Rob said, the console.log/console.warn/console.err methods are just writers to stdout/stderr, there's nothing that uses those method names to determine if something is outputted or not. — Joe
– Joe, Commented Sep 22, 2012 at 13:20
thanks guys, i'm using this library now in case anyone is interested: github.com/visionmedia/log.js — kocodude
– kocodude, Commented Oct 1, 2012 at 5:24
@kocodude but how do you filter the logs you see from log.js based on an env variable - as per your question ? ( I have the same question ) stackoverflow.com/questions/71082365/… — George Katsanos
– George Katsanos, Commented Feb 11, 2022 at 15:20

Ayes · Accepted Answer · 2024-04-24 16:40:24Z

You can overwrite the console.log,console.debug,console.error,console.warn functions to allow logLevels.

Check out my snippet written in typescript:

export const logLevels = ["debug", "log", "warn", "error", "none"] as const;
type LogLevel = (typeof logLevels)[number];

declare global {
  var logLevel: LogLevel;
}

const shouldLog = (level: LogLevel) => {
  return logLevels.indexOf(level) >= logLevels.indexOf(global.logLevel);
};

global.logLevel = "debug";

const _console = console
global.console = {
  ...global.console,
  log: (message?: any, ...optionalParams: any[]) => {
    shouldLog("log") && _console.log(message, ...optionalParams);
  },
  warn: (message?: any, ...optionalParams: any[]) => {
    shouldLog("warn") && _console.warn(message, ...optionalParams);
  },
  error: (message?: any, ...optionalParams: any[]) => {
    shouldLog("error") && _console.error(message, ...optionalParams);
  },
  debug: (message?: any, ...optionalParams: any[]) => {
    shouldLog("debug") && _console.debug(message, ...optionalParams);
  },
};

Then you can use the console functions as usual and set the logLevel with globals.logLevel="warn"

AndyA · Accepted Answer · 2015-07-29 13:10:33Z

2

Not possibly quite what you are after but you can enable debugging within node by setting a NODE_DEBUG environment variable first.

E.g. export NODE_DEBUG="net" && node myapp.js will give debugging for network related node operations.

https://github.com/joyent/node/blob/master/lib/net.js#L66

answered Jul 29, 2015 at 13:10

AndyA

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