Structured logging with JSON in Express

Introduction

Structured logging with JSON helps you keep logs neat and easy to read by machines and people. It makes finding problems faster and sharing logs simpler.

When you want to track user actions in a web app clearly.

When you need to send logs to a monitoring system that reads JSON.

When debugging issues and you want detailed, organized log info.

When multiple developers or teams need to understand logs easily.

When logs must be searchable and filterable by keys like user ID or error type.

Syntax

Express

console.log(JSON.stringify({ key1: 'value1', key2: 'value2' }))

Use JSON.stringify() to convert an object into a JSON string for logging.

Include keys that describe the event, like level, message, and timestamp.

Examples

Logs a simple info message about the server starting.

Express

console.log(JSON.stringify({ level: 'info', message: 'Server started' }))

Logs an error with a code to help identify the problem.

Express

console.log(JSON.stringify({ level: 'error', message: 'Failed to connect', errorCode: 500 }))

Logs a debug message with user ID and timestamp for tracking.

Express

console.log(JSON.stringify({ level: 'debug', message: 'User login', userId: 123, time: new Date().toISOString() }))

Sample Program

This Express app logs every incoming request as a JSON string with details like method, URL, and time. It also logs when the server starts.

Express

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

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

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

app.listen(3000, () => {
  console.log(JSON.stringify({ level: 'info', message: 'Server listening on port 3000', timestamp: new Date().toISOString() }));
});

Important Notes

Make sure to include timestamps to know when events happen.

Use consistent keys in your JSON logs for easier searching and filtering.

Consider using logging libraries like winston or pino for more features.

Summary

Structured logging uses JSON to keep logs clear and organized.

It helps machines and people understand logs quickly.

Include useful info like level, message, and timestamp in every log.

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