Structured logging with JSON in Express - Mini Project: Build & Apply

Structured logging with JSON in Express

📖 Scenario: You are building a simple Express server. To help with debugging and monitoring, you want to log requests in a structured JSON format. This makes logs easier to read and analyze by tools.

🎯 Goal: Create an Express server that logs each incoming request as a JSON object with keys for method, url, and timestamp.

📋 What You'll Learn

Create an Express app instance

Add a middleware to log requests in JSON format

Log method, url, and timestamp in ISO format

Start the server on port 3000

Print the JSON log to the console for each request

💡 Why This Matters

🌍 Real World

Structured JSON logs help developers and operations teams quickly understand server activity and diagnose issues by parsing logs with tools.

💼 Career

Logging is a key skill for backend developers and DevOps engineers to monitor and troubleshoot applications in production.

1

Setup Express app

Create a variable called express that requires the 'express' module. Then create a variable called app by calling express().

Express

// Your code here

Hint

Use require('express') to import Express, then call it as a function to create the app.

2

Add JSON logging middleware

Add a middleware to app using app.use. The middleware should be a function with parameters req, res, and next. Inside it, create a variable logEntry that is an object with keys method (from req.method), url (from req.url), and timestamp (current time in ISO string format). Then log logEntry as a JSON string using console.log(JSON.stringify(logEntry)). Finally, call next().

Express

const express = require('express');
const app = express();

// Add logging middleware here
// Your code here

Hint

The middleware function logs the request details as JSON and then calls next() to continue.

3

Add a simple route

Add a GET route on / to app using app.get. The route handler should send the text 'Hello World' as the response.

Express

const express = require('express');
const app = express();

app.use((req, res, next) => {
    const logEntry = {
        method: req.method,
        url: req.url,
        timestamp: new Date().toISOString()
    };
    console.log(JSON.stringify(logEntry));
    next();
});

// Add GET route for '/' here
// Your code here

Hint

Use app.get with path '/' and send 'Hello World' as response.

4

Start the server

Start the Express server by calling app.listen on port 3000. Add a callback function that prints 'Server running on port 3000' to the console.

Express

const express = require('express');
const app = express();

app.use((req, res, next) => {
    const logEntry = {
        method: req.method,
        url: req.url,
        timestamp: new Date().toISOString()
    };
    console.log(JSON.stringify(logEntry));
    next();
});

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

// Start server on port 3000
// Your code here

Hint

Use app.listen(3000, () => { console.log(...) }) to start the server.

Practice

1. What is the main benefit of using structured logging with JSON in an Express app?

easy

A. It makes logs easy to read and parse by both humans and machines.

B. It hides all log messages from users.

C. It automatically fixes bugs in the code.

D. It slows down the server to improve security.

Solution

1. Step 1: Understand structured logging purpose

   Structured logging formats logs in a clear, consistent way using JSON.

2. Step 2: Identify benefits of JSON logs

   JSON logs are easy for both humans and machines to read and analyze quickly.

3. Final Answer:

   It makes logs easy to read and parse by both humans and machines. -> Option A

4. Quick Check:

   Structured logging = clear, readable logs [OK]

Hint: Think: JSON logs help both people and tools read logs fast [OK]

Common Mistakes:

• Confusing structured logging with hiding logs
• Assuming it fixes code bugs automatically
• Thinking it slows server for security

2. Which of the following is the correct way to log a JSON object with level and message in Express using console.log?

easy

A. console.log('{ level: "info", message: "Server started" }');

B. console.log({ level: 'info', message: 'Server started' });

C. console.log('level: info, message: Server started');

D. console.log(level = 'info', message = 'Server started');

Solution

1. Step 1: Check JSON object syntax in JavaScript

   JSON objects in JS use curly braces with key-value pairs, keys as strings or identifiers, values as strings or variables.

2. Step 2: Identify correct console.log usage

   console.log accepts an object directly to print it as JSON-like output, so console.log({ level: 'info', message: 'Server started' }); is correct.

3. Final Answer:

   console.log({ level: 'info', message: 'Server started' }); -> Option B

4. Quick Check:

   Use object syntax inside console.log [OK]

Hint: Use curly braces {} for objects inside console.log [OK]

Common Mistakes:

• Passing JSON as a string instead of object
• Using incorrect syntax with equals inside console.log
• Logging plain strings without JSON structure

3. Given the code snippet:

const log = (level, message) => {
  console.log(JSON.stringify({ level, message, timestamp: new Date().toISOString() }));
};
log('error', 'Failed to connect');

What will be the output format?

medium

A. Error: Failed to connect

B. level: error, message: Failed to connect, timestamp: 2024-06-01T12:00:00.000Z

C. [error] Failed to connect at 2024-06-01T12:00:00.000Z

D. {"level":"error","message":"Failed to connect","timestamp":"2024-06-01T12:00:00.000Z"}

Solution

1. Step 1: Understand JSON.stringify output

   JSON.stringify converts the object into a JSON string with keys and values in double quotes.

2. Step 2: Analyze the log function output

   The log function prints a JSON string with keys level, message, and timestamp in ISO format.

3. Final Answer:

   {"level":"error","message":"Failed to connect","timestamp":"2024-06-01T12:00:00.000Z"} -> Option D

4. Quick Check:

   JSON.stringify outputs JSON string [OK]

Hint: JSON.stringify outputs double-quoted JSON string [OK]

Common Mistakes:

• Expecting plain text instead of JSON string
• Confusing JSON string with object literal
• Ignoring timestamp format

4. You wrote this code to log JSON in Express:

console.log(JSON.stringify(level: 'info', message: 'Started'));

What is the error and how to fix it?

medium

A. TypeError because console.log cannot print JSON strings; fix by removing JSON.stringify.

B. No error; code logs JSON correctly.

C. SyntaxError due to incorrect JSON.stringify argument; fix by passing an object inside curly braces.

D. ReferenceError because level and message are undefined variables.

Solution

1. Step 1: Identify JSON.stringify argument syntax

   JSON.stringify expects a single object or value; here, the argument uses invalid syntax without braces.

2. Step 2: Correct the argument to an object

   Wrap the key-value pairs inside curly braces to form a valid object: { level: 'info', message: 'Started' }.

3. Final Answer:

   SyntaxError due to incorrect JSON.stringify argument; fix by passing an object inside curly braces. -> Option C

4. Quick Check:

   JSON.stringify needs an object argument [OK]

Hint: Always pass an object inside {} to JSON.stringify [OK]

Common Mistakes:

• Passing key-value pairs without braces
• Assuming JSON.stringify accepts multiple arguments
• Ignoring syntax errors in object literals

5. You want to add structured logging in Express that includes level, message, timestamp, and requestId from middleware. Which code snippet correctly logs this info?

hard

A. app.use((req, res, next) => {
console.log(JSON.stringify({ level: 'info', message: 'Request received', timestamp: new Date().toISOString(), requestId: req.id }));
next();
});

B. app.use((req, res, next) => {
console.log({ level: 'info', message: 'Request received', timestamp: new Date(), requestId: req.id });
next();
});

C. app.use((req, res, next) => {
console.log('Request received at ' + new Date() + ' with id ' + req.id);
next();
});

D. app.use((req, res, next) => {
console.log(JSON.stringify(level: 'info', message: 'Request received', timestamp: new Date().toISOString(), requestId: req.id));
next();
});

Solution

1. Step 1: Check correct JSON object syntax inside JSON.stringify

   app.use((req, res, next) => { console.log(JSON.stringify({ level: 'info', message: 'Request received', timestamp: new Date().toISOString(), requestId: req.id })); next(); }); correctly passes an object with keys and values inside curly braces to JSON.stringify.

2. Step 2: Verify inclusion of all required fields and middleware usage

   app.use((req, res, next) => { console.log(JSON.stringify({ level: 'info', message: 'Request received', timestamp: new Date().toISOString(), requestId: req.id })); next(); }); includes level, message, timestamp (ISO string), and requestId from req.id, then calls next() properly.

3. Final Answer:

   app.use((req, res, next) => { console.log(JSON.stringify({ level: 'info', message: 'Request received', timestamp: new Date().toISOString(), requestId: req.id })); next(); }); -> Option A

4. Quick Check:

   Middleware logs JSON object with all fields [OK]

Hint: Use JSON.stringify with object including all fields inside middleware [OK]

Common Mistakes:

• Passing invalid syntax to JSON.stringify
• Logging plain strings instead of JSON
• Forgetting to call next() in middleware