8754. Logging with Winston
Express and Winston

Introduce how to use Winston for logging in express server.

1. Winston

Winston a versatile async logging library for Node.js.

1.1 Logging Levels

Winston provides the following default log levels. They are prioritized from 0 to 5 (highest to lowest)

const levels = { error: 0, warn: 1, info: 2, verbose: 3, debug: 4, silly: 5 } 

You can pass a string representing the logging level to the log() method or use the level specified methods defined on every winston logger.

logger.log('info', "hello world!"); logger.log('error', "Ops, you got an error!"); // equivalent to logger.info("hello world!"); logger.error("Ops, you got an error!"); // same output // info: hello world! // error: Ops, you got an error! 

1.2 Transports

A transport is essentially a storage device for your logs. Each Winston logger can have multiple transports configured at different levels. Winston provides two default transports, Console and File.

transports: [ new winston.transports.Console(), new winston.transports.File({ filename: 'combined.log' }) ] 

1.3 Multiple Transports of Same Type

It is possible to use multiple transports of the same type e.g. winston.transports.File when you construct the transport.

const logger = winston.createLogger({ transports: [ new winston.transports.File({ filename: 'combined.log', level: 'info' }), new winston.transports.File({ filename: 'errors.log', level: 'error' }) ] }); 

2. Examples

2.1 Print Logs to Console

Create winston logger.

//winston-config-console.js var path = require("path"); var winston = require("winston"); const logger = winston.createLogger({ transports: [ new winston.transports.Console({ colorize: true }) ] }); module.exports = logger; 

Express server.

// server.js var express = require("express"); var logger = require("./config/winston-config-console"); var app = express(); app.get("/", function(req, res) { logger.info("Hello world"); res.send("hello, world!"); }); app.use(function(req, res) { logger.error("File not found"); res.status(404).send("File not found!"); }); app.listen(3000, function() { console.log("Web Server started on port 3000"); }); 

Start the server and access http://localhost:3000/ and http://localhost:3000/random in browser. You should see the output in console.

Web Server started on port 3000 {"message":"File not found","level":"error"} {"message":"Hello world","level":"info"}

Add format to transport.

const logger = winston.createLogger({ transports: [ new winston.transports.Console({ format: winston.format.simple(), colorize: true }) ] }); 

Try to access those two URLs again. This time you would see the following logs.

Web Server started on port 3000 info: Hello world error: File not found

2.2 Write Logs to File

Create another winston logger.

//winston-config-file.js var path = require("path"); var fs = require("fs"); var appRoot = require("app-root-path"); var winston = require("winston"); // ensure log directory exists var logDirectory = path.resolve(`${appRoot}`, "logs"); fs.existsSync(logDirectory) || fs.mkdirSync(logDirectory); var options = { infofile: { level: "info", filename: path.resolve(logDirectory, "info.log"), handleExceptions: true, json: true, maxsize: 5242880, // 5MB maxFiles: 5 }, errorfile: { level: "error", filename: path.resolve(logDirectory, "error.log"), handleExceptions: true, json: true, maxsize: 5242880, // 5MB maxFiles: 5 } }; const logger = winston.createLogger({ transports: [ new winston.transports.File(options.infofile), new winston.transports.File(options.errorfile) ] }); module.exports = logger; 

Update server.js with the new winston logger.

// server.js var express = require("express"); //var logger = require("./config/winston-config-console"); var logger = require("./config/winston-config-file"); ... 

Start the server and access http://localhost:3000/ and http://localhost:3000/random in browser. This time you won’t see any log in console. Instead, you should see the following logs in ./logs/error.log.

{"message":"File not found","level":"error"}

And the following logs in ./logs/info.log

{"message":"Hello world","level":"info"} {"message":"File not found","level":"error"}

Notice that, the error log is also recorded. This is because, for level ‘info’, all level value less or equals 2 will be logged. Logs for level ‘error: 0’, ‘warn: 1’ and ‘info: 2’ are all saved to this file.

2.3 Log File Rotation

Winston supports to log rotation by winston-daily-rotate-file. The DailyRotateFile transport can rotate files by minute, hour, day, month, year or weekday.

//winston-config-rotate.js var path = require("path"); var fs = require("fs"); var appRoot = require("app-root-path"); var winston = require("winston"); require("winston-daily-rotate-file"); // ensure log directory exists var logDirectory = path.resolve(`${appRoot}`, "logs"); fs.existsSync(logDirectory) || fs.mkdirSync(logDirectory); var transport = new winston.transports.DailyRotateFile({ filename: path.resolve(logDirectory, "application-%DATE%.log"), datePattern: "YYYY-MM-DD-HH", zippedArchive: true, maxSize: "20m", maxFiles: "14d" // keep logs for 14 days }); transport.on("rotate", function(oldFilename, newFilename) { // do something fun }); const logger = winston.createLogger({ transports: [transport] }); module.exports = logger; 