Error handling and logging in Deno - Mini Project: Build & Apply

Error handling and logging

📖 Scenario: You are building a simple Deno script that reads user data from a JSON file. Sometimes the file might be missing or contain invalid data. You want to handle these errors gracefully and log useful messages to understand what went wrong.

🎯 Goal: Create a Deno script that reads a JSON file named user.json, handles errors if the file is missing or the JSON is invalid, and logs messages for success or failure.

📋 What You'll Learn

Create a variable to hold the filename user.json

Add a variable to hold a logger prefix string

Use a try-catch block to read and parse the JSON file

Log success or error messages with the prefix

Print the parsed user data if successful

💡 Why This Matters

🌍 Real World

Handling errors and logging is essential in real-world scripts and applications to avoid crashes and to understand issues quickly.

💼 Career

DevOps engineers and developers must write robust scripts that handle failures gracefully and provide clear logs for troubleshooting.

1

Setup filename variable

Create a variable called filename and set it to the string "user.json".

Deno

# Create a variable called filename and set it to "user.json"
# Your code here

Hint

Use const filename = "user.json"; to store the file name.

2

Add logger prefix

Add a variable called logPrefix and set it to the string "[UserLoader]".

Deno

const filename = "user.json";
# Add a variable called logPrefix and set it to "[UserLoader]"
# Your code here

Hint

Use const logPrefix = "[UserLoader]"; to create a prefix for log messages.

3

Read and parse JSON with error handling

Use a try-catch block to read the file filename using Deno.readTextFile, parse it with JSON.parse, and store the result in a variable called userData. In the catch block, log an error message using console.error with the logPrefix and the error message.

Deno

const filename = "user.json";
const logPrefix = "[UserLoader]";

// Use try-catch to read and parse JSON
// Your code here

Hint

Use try { const text = await Deno.readTextFile(filename); userData = JSON.parse(text); } and catch (error) { console.error(`${logPrefix} Error loading user data: ${error.message}`); }.

4

Print the user data

Add a console.log statement to print the variable userData.

Deno

const filename = "user.json";
const logPrefix = "[UserLoader]";

let userData;
try {
  const text = await Deno.readTextFile(filename);
  userData = JSON.parse(text);
  console.log(`${logPrefix} Successfully loaded user data.`);
} catch (error) {
  console.error(`${logPrefix} Error loading user data: ${error.message}`);
}

// Print the userData variable
# Your code here

Hint

Use console.log(userData); to display the loaded data.

Practice

1. What is the main purpose of using try...catch in Deno?

easy

A. To handle errors and keep the program running smoothly

B. To speed up the program execution

C. To create new files automatically

D. To log messages to the console

Solution

1. Step 1: Understand the role of try...catch

   The try...catch block is used to catch errors that happen during program execution.

2. Step 2: Identify the effect on program flow

   By catching errors, the program can handle them gracefully and continue running instead of crashing.

3. Final Answer:

   To handle errors and keep the program running smoothly -> Option A

4. Quick Check:

   Error handling = To handle errors and keep the program running smoothly [OK]

Hint: Try...catch blocks catch errors to prevent crashes [OK]

Common Mistakes:

• Thinking try...catch speeds up code
• Confusing error handling with logging
• Believing try...catch creates files

2. Which of the following is the correct syntax to log an error message in Deno?

easy

A. console.log.error('An error occurred')

B. console.error('An error occurred')

C. console.err('An error occurred')

D. console.errorMessage('An error occurred')

Solution

1. Step 1: Recall the console methods in Deno

   Deno uses standard JavaScript console methods like console.error() to log error messages.

2. Step 2: Check each option's syntax

   Only console.error('An error occurred') is valid syntax for logging errors.

3. Final Answer:

   console.error('An error occurred') -> Option B

4. Quick Check:

   console.error() logs errors = console.error('An error occurred') [OK]

Hint: Use console.error() to log errors clearly [OK]

Common Mistakes:

• Using non-existent console methods
• Mixing console.log and error methods incorrectly
• Misspelling console.error

3. What will be the output of this Deno code?

try {
  throw new Error('Oops!')
} catch (e) {
  console.log('Caught:', e.message)
}

medium

A. Caught: Oops!

B. Error: Oops!

C. Caught: undefined

D. No output

Solution

1. Step 1: Understand the throw and catch behavior

   The code throws an error with message 'Oops!'. The catch block catches it and accesses e.message.

2. Step 2: Analyze the console.log output

   The catch block logs 'Caught:' followed by the error message, so output is 'Caught: Oops!'.

3. Final Answer:

   Caught: Oops! -> Option A

4. Quick Check:

   Catch logs error message = Caught: Oops! [OK]

Hint: Catch block logs error.message after throw [OK]

Common Mistakes:

• Expecting full error object printed
• Confusing error message with undefined
• Thinking no output occurs

4. Identify the error in this Deno code snippet:

try {
  const data = JSON.parse('{ bad json }')
} catch {
  console.log('Parsing failed', e.message)
}

medium

A. console.log should be console.error

B. JSON.parse syntax is correct

C. Missing error parameter in catch block

D. try block should not have code

Solution

1. Step 1: Check catch block syntax

   In Deno (and modern JavaScript), catch block can omit error parameter, but to access error details, it must be declared.

2. Step 2: Identify the problem with missing error parameter

   The code tries to catch error but does not declare error variable, so error details can't be accessed if needed.

3. Final Answer:

   Missing error parameter in catch block -> Option C

4. Quick Check:

   Catch needs error param to access error = Missing error parameter in catch block [OK]

Hint: Always include error param in catch to access error info [OK]

Common Mistakes:

• Assuming catch() always requires error param
• Thinking console.log is wrong for errors
• Believing try block can't have code

5. You want to log an error with a timestamp and the error message in Deno. Which code snippet correctly does this?

hard

A. try {
throw new Error('Fail')
} catch (e) {
console.log('Error:', e)
}

B. try {
throw new Error('Fail')
} catch (e) {
console.error(e)
console.log(new Date())
}

C. try {
throw new Error('Fail')
} catch {
console.error('Error occurred')
}

D. try {
throw new Error('Fail')
} catch (e) {
console.error(`[${new Date().toISOString()}] Error: ${e.message}`)
}

Solution

1. Step 1: Understand the requirement for timestamp and message

   The log must include current time in ISO format and the error message.

2. Step 2: Check each option for correct formatting

   try { throw new Error('Fail') } catch (e) { console.error(`[${new Date().toISOString()}] Error: ${e.message}`) } uses template literals to combine timestamp and error message in one console.error call, which is correct.

3. Final Answer:

   try { throw new Error('Fail') } catch (e) { console.error(`[${new Date().toISOString()}] Error: ${e.message}`) } -> Option D

4. Quick Check:

   Timestamp + error message in one log = try { throw new Error('Fail') } catch (e) { console.error(`[${new Date().toISOString()}] Error: ${e.message}`) } [OK]

Hint: Use template literals with new Date().toISOString() for timestamps [OK]

Common Mistakes:

• Logging error and timestamp separately
• Missing error parameter in catch
• Using console.log instead of console.error for errors