Log levels and when to use them in Express - Mini Project: Build & Apply

Log levels and when to use them in Express

📖 Scenario: You are building a simple Express server that logs messages at different levels to help track what happens during requests. Logs help developers understand the app's behavior and find problems.

🎯 Goal: Create an Express app that uses different log levels (info, warn, error) to show messages for normal operations, warnings, and errors.

📋 What You'll Learn

Create an Express app with a single route

Set up a variable for the current log level

Use console.info for informational messages

Use console.warn for warning messages

Use console.error for error messages

Log messages only if their level is equal or higher priority than the current log level

💡 Why This Matters

🌍 Real World

Logging is essential in real-world web servers to monitor app behavior, catch errors early, and understand user activity.

💼 Career

Knowing how to use log levels helps developers write maintainable and debuggable server code, a key skill for backend development jobs.

1

Set up Express app and route

Create an Express app by importing express and calling express(). Then add a GET route at '/' that sends the text 'Hello, world!' as the response.

Express

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

// Your code here

Hint

Use require('express') to import Express. Then create the app with express(). Add a GET route with app.get('/', (req, res) => { ... }).

2

Add a log level variable

Create a variable called currentLogLevel and set it to the string 'info'. This will control which log messages show.

Express

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

const currentLogLevel = 'info';
// Your code here

Hint

Just create a constant named currentLogLevel and assign it the string 'info'.

3

Add logging with levels inside the route

Inside the GET route, add three if statements that check currentLogLevel to decide which logs to show. Use console.info('Info: Request received') if currentLogLevel is 'info'. Use console.warn('Warning: Slow response') if currentLogLevel is 'warn' or 'info'. Use console.error('Error: Something went wrong') if currentLogLevel is 'error', 'warn', or 'info'. This simulates showing logs only if their level is important enough.

Express

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

const currentLogLevel = 'info';

app.get('/', (req, res) => {
  // Add log level checks and console logs here
  // Your code here
  res.send('Hello, world!');
});

Hint

Use if statements to check currentLogLevel and call the right console method for each log level.

4

Start the server listening on port 3000

Add app.listen(3000) to start the Express server on port 3000.

Express

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

const currentLogLevel = 'info';

app.get('/', (req, res) => {
  if (currentLogLevel === 'info') {
    console.info('Info: Request received');
  }
  if (currentLogLevel === 'info' || currentLogLevel === 'warn') {
    console.warn('Warning: Slow response');
  }
  if (currentLogLevel === 'info' || currentLogLevel === 'warn' || currentLogLevel === 'error') {
    console.error('Error: Something went wrong');
  }
  res.send('Hello, world!');
});

// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

Practice

1. Which log level in Express is best used to record normal operation messages that confirm things are working as expected?

easy

A. info

B. error

C. debug

D. warn

Solution

1. Step 1: Understand the purpose of log levels

   Log levels help separate messages by importance. 'info' is for normal operations.

2. Step 2: Match log level to message type

   'info' is used to confirm the app is running fine, unlike 'error' or 'warn' which indicate problems.

3. Final Answer:

   info -> Option A

4. Quick Check:

   Normal operation messages = info [OK]

Hint: Info logs show normal app activity clearly [OK]

Common Mistakes:

• Confusing 'info' with 'error'
• Using 'debug' for normal messages
• Thinking 'warn' is for normal info

2. Which of the following is the correct way to log a warning message using Express's console object?

easy

A. console.info('This is a warning')

B. console.error('This is a warning')

C. console.debug('This is a warning')

D. console.warn('This is a warning')

Solution

1. Step 1: Identify the correct console method for warnings

   Express uses standard console methods; 'warn' is for warnings.

2. Step 2: Match method to message type

   console.warn() logs warning messages, unlike console.error() which logs errors.

3. Final Answer:

   console.warn('This is a warning') -> Option D

4. Quick Check:

   Warning messages use console.warn() [OK]

Hint: Use console.warn() for warnings, not error or info [OK]

Common Mistakes:

• Using console.error for warnings
• Using console.info for warnings
• Using console.debug for warnings

3. What will be the output of this Express logging code snippet?

console.info('Server started');
console.warn('Memory usage high');
console.error('Failed to connect to DB');
console.debug('User ID: 1234');

medium

A. Only 'Failed to connect to DB' will appear

B. Only 'Server started' and 'Memory usage high' will appear

C. All four messages will appear in the console

D. Only 'User ID: 1234' will appear

Solution

1. Step 1: Understand default console behavior in Express

   By default, all console methods (info, warn, error, debug) print messages to the console.

2. Step 2: Analyze the code snippet output

   All four console calls will output their messages unless filtered by a logging library, which is not shown here.

3. Final Answer:

   All four messages will appear in the console -> Option C

4. Quick Check:

   Default console logs all levels [OK]

Hint: Default console logs all messages unless filtered [OK]

Common Mistakes:

• Thinking debug logs are hidden by default
• Assuming error logs suppress others
• Confusing info and warn visibility

4. You want to log an error in Express but accidentally write console.warn('Error occurred'). What is the main issue with this?

medium

A. The code will cause a syntax error

B. The message will be logged as a warning, not an error

C. The message will not appear in the console

D. The app will crash

Solution

1. Step 1: Identify the difference between warn and error logs

   console.warn logs warnings; console.error logs errors. They differ in severity.

2. Step 2: Understand the impact of using warn instead of error

   Using console.warn will log the message but mark it as a warning, not an error, which may mislead debugging.

3. Final Answer:

   The message will be logged as a warning, not an error -> Option B

4. Quick Check:

   Wrong log level changes message severity [OK]

Hint: Use console.error for errors, not console.warn [OK]

Common Mistakes:

• Thinking warn causes errors
• Expecting syntax error
• Assuming message won't show

5. In an Express app, you want to log detailed information only when debugging is enabled, but always log warnings and errors. Which approach correctly uses log levels to achieve this?

hard

A. Use console.debug for detailed info, console.warn for warnings, and console.error for errors; enable debug logs conditionally.

B. Use console.info for all logs and filter messages manually.

C. Use console.error for all messages to ensure visibility.

D. Use console.warn for detailed info and console.debug for errors.

Solution

1. Step 1: Assign log levels by message importance

   Detailed info fits debug, warnings fit warn, and errors fit error.

2. Step 2: Enable debug logs conditionally

   Debug logs can be turned on or off based on environment or settings, while warnings and errors always log.

3. Final Answer:

   Use console.debug for detailed info, console.warn for warnings, and console.error for errors; enable debug logs conditionally. -> Option A

4. Quick Check:

   Conditional debug, always warn/error = correct logging [OK]

Hint: Debug logs conditional; warn and error always on [OK]

Common Mistakes:

• Logging all as info
• Using error for all messages
• Mixing warn and debug levels incorrectly