Morgan HTTP Logger

1.10.1 · active · verified Sat Apr 18

Morgan is an HTTP request logger middleware for Node.js, often used with the Express framework. It allows developers to log request details such as method, URL, status, and response time in various predefined or custom formats. The current stable version is 1.10.1. Releases occur infrequently, with the latest update focusing on minor fixes and dependency updates, indicating a mature and stable library.

Common errors

```
SyntaxError: Cannot use import statement outside a module
```
cause Attempting to use ES module `import` syntax (`import morgan from 'morgan'`) in a CommonJS-only Node.js environment or for this CommonJS-only package.

fix Use the CommonJS `require` syntax: `const morgan = require('morgan');`
```
TypeError: app.use() requires a middleware function but got a Object
```
cause This error typically occurs in Express.js when you pass `morgan` (the module itself) directly to `app.use()` instead of calling it to get the middleware function (e.g., `morgan('dev')`).

fix Call `morgan` with a format string or custom function to create the middleware, for example: `app.use(morgan('dev'));`

Warnings

gotcha When using the `immediate: true` option, logs are written on request instead of response. This means response-specific data (like `:status` or `:res[content-length]`) will not be available in the log line, or may appear as `undefined`.
Fix: Only use `immediate: true` if you specifically need logs to be written even if the server crashes before sending a response, and you don't require response-specific tokens. Otherwise, omit the option for default behavior.
deprecated The `DEBUG_FD` environment variable, used for debugging output stream, was deprecated in `morgan` v1.8.0.
Fix: Avoid using `DEBUG_FD`. Consult the `debug` package documentation for alternative debugging configurations.
breaking The behavior of the `:response-time` token changed in v1.6.0. Previously, it might have included response latency (the time taken to *send* the response). From v1.6.0 onwards, it strictly measures the time from request start to when response headers are sent.
Fix: If your application relies on the old `:response-time` behavior, be aware of this change and adjust your logging interpretation or custom token implementation accordingly. Upgrade to v1.6.0+ for the more accurate measurement.
gotcha Prior to v1.9.1, using certain special characters in custom format strings could lead to incorrect logging or parsing issues.
Fix: Upgrade to `morgan` v1.9.1 or later to ensure proper handling of special characters in format strings.

Install

npm install morgan npm
yarn add morgan yarn
pnpm add morgan pnpm

Quickstart

This quickstart sets up a basic Express.js application and integrates Morgan middleware using the 'dev' format for logging HTTP requests. It demonstrates how Morgan logs incoming requests and their responses, including status codes and response times.

const express = require('express');
const morgan = require('morgan');
const app = express();
const port = process.env.PORT ?? 3000;

// Use 'dev' format for concise colored output during development
app.use(morgan('dev'));

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.get('/error', (req, res) => {
  res.status(500).send('Something broke!');
});

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);
});

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