morgan

Install the morgan HTTP request logger middleware using the npm registry.

$ npm install morgan

The options object in morgan(format, options) accepts the following properties:

• immediate: (Boolean) Write the log line on request instead of response. Note that response data (like status code or content length) will not be available.
• skip: (Function) A function skip(req, res) that returns true to skip logging.
• stream: (Stream) The output stream for writing log lines. Defaults to process.stdout.

// Example: only log error responses
morgan('combined', {
  skip: function (req, res) { return res.statusCode < 400 }
})

To use morgan in an Express application, require it and pass the desired format to app.use().

var express = require('express')
var morgan = require('morgan')

var app = express()

app.use(morgan('combined'))

app.get('/', function (req, res) {
  res.send('hello, world!')
})

To log to a file instead of stdout, create a write stream using Node's fs module and pass it to the stream option in morgan.

var express = require('express')
var fs = require('fs')
var morgan = require('morgan')
var path = require('path')

var app = express()

// create a write stream (in append mode)
var accessLogStream = fs.createWriteStream(path.join(__dirname, 'access.log'), { flags: 'a' })

// setup the logger
app.use(morgan('combined', { stream: accessLogStream }))

app.get('/', function (req, res) {
  res.send('hello, world!')
})

You can define new tokens by calling morgan.token(name, callback). The callback function is called with req and res and should return a string. Calling morgan.token() with an existing name will overwrite the previous definition.

morgan.token('type', function (req, res) { return req.headers['content-type'] })

Create a new morgan logger middleware function. The format argument can be a predefined name (string), a custom format string using tokens, or a function that produces a log entry.

Format Function Arguments:

• tokens: An object containing all defined tokens.
• req: The HTTP request object.
• res: The HTTP response object.

The function should return a string for the log line, or undefined/null to skip logging.

Morgan provides several built-in format strings:

• combined: Standard Apache combined log output.
• common: Standard Apache common log output.
• dev: Concise, color-coded output for development (status codes are colored).
• short: A shorter format including response time.
• tiny: The minimal output format.

Tokens are referenced in format strings using the : prefix.

Standard Tokens:

• :date[format]: Current UTC date/time. Formats: clf, iso, or web (default).
• :http-version: The HTTP version of the request.
• :method: The HTTP method.
• :pid: The Node.js process ID.
• :referrer: The Referrer header.
• :remote-addr: The remote address (req.ip or socket address).
• :remote-user: Authenticated user (Basic auth).
• :req[header]: A specific request header.
• :res[header]: A specific response header.
• :response-time[digits]: Time between request and response headers written (ms). digits defaults to 3.
• :status: The HTTP response status code.
• :total-time[digits]: Time between request and response finished (ms). digits defaults to 3.
• :url: The request URL.
• :user-agent: The User-Agent header.

You can extend morgan's logging capabilities by defining custom tokens using morgan.token(name, fn). The function fn receives (req, res, [args]) and should return the string value to be logged.

Example of defining a custom token:

Use morgan.format(name, fmt) to register a new named format. This allows you to use the name as a string in the morgan() middleware factory.

• name: The string identifier for the format.
• fmt: A format string or a function that generates the log line.

Use morgan(format, options) to create an HTTP request logger middleware.

• format: A string representing a predefined format (like 'dev', 'combined', 'tiny'), a custom format string with tokens, or a function.
• options: An optional configuration object:
  • immediate: If true, logs the request immediately instead of waiting for the response to finish.
  • skip: A function (req, res) => boolean that determines if a log entry should be skipped.
  • stream: A writable stream to which logs are written (defaults to process.stdout).
  • buffer: A number representing the buffer duration in milliseconds. If provided, logs are buffered and flushed at intervals.