Logging

By default, Bolt for JavaScript will log information from your app to the console. You can customize how much logging occurs by passing a logLevel in the constructor. The available log levels in order of most to least logs are DEBUG, INFO, WARN, and ERROR.

// Import LogLevel from the package
const { App, LogLevel } = require('@slack/bolt');

// Log level is one of the options you can set in the constructor
const app = new App({
  token,
  signingSecret,
  logLevel: LogLevel.DEBUG,
});

Writing logs

The logger included with the constructed App can be used to log writings throughout your application code:

(async () => {
  app.logger.debug("Starting the app now!");
  await app.start();
  app.logger.info("⚡️ Bolt app started");
})();

Different app listeners can use the same logger that's provided as an argument to output additional details:

app.event("team_join", async ({ client, event, logger }) => {
  logger.info("Someone new just joined the team.");
  try {
    const result = await client.chat.postMessage({
      channel: "C0123456789",
      text: `Welcome to the team, <@${event.user.id}>!`,
    });
    logger.debug(result);
  } catch (error) {
    logger.error(error);
  }
});

Redirecting outputs

If you want to send logs to somewhere besides the console or want more control over the logger, you can implement a custom logger. A custom logger must implement specific methods (known as the Logger interface):

Method	Parameters	Return type
setLevel()	level: LogLevel	void
getLevel()	None	string with value error, warn, info, or debug
setName()	name: string	void
debug()	...msgs: any[]	void
info()	...msgs: any[]	void
warn()	...msgs: any[]	void
error()	...msgs: any[]	void

A very simple custom logger might ignore the name and level, and write all messages to a file.

const { App } = require('@slack/bolt');
const { createWriteStream } = require('fs');
const logWritable = createWriteStream('/var/my_log_file'); // Not shown: close this stream

const app = new App({
  token,
  signingSecret,
  // Creating a logger as a literal object. It's more likely that you'd create a class.
  logger: {
    debug: (...msgs) => { logWritable.write('debug: ' + JSON.stringify(msgs)); },
    info: (...msgs) => { logWritable.write('info: ' + JSON.stringify(msgs)); },
    warn: (...msgs) => { logWritable.write('warn: ' + JSON.stringify(msgs)); },
    error: (...msgs) => { logWritable.write('error: ' + JSON.stringify(msgs)); },
    setLevel: (level) => { },
    getLevel: () => { },
    setName: (name) => { },
  },
});