Error handling and logging in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Deno error handling code?

Consider this Deno script that tries to read a file and logs errors. What will it output if the file does not exist?

Deno

try {
  const data = await Deno.readTextFile("nofile.txt");
  console.log(data);
} catch (error) {
  console.error("Error reading file:", error.message);
}

A File content printed

B undefined

C SyntaxError: Unexpected token

D Error reading file: No such file or directory (os error 2)

💡 Hint

Think about what happens when you try to read a file that does not exist in Deno.

When Deno.readTextFile tries to read a non-existent file, it throws an error. The catch block logs the error message, which includes 'No such file or directory'.

🧠 Conceptual

intermediate

Which Deno API is best for structured logging with levels?

You want to add logs with levels like info, warning, and error in a Deno application. Which built-in or standard module should you use?

A Use the std/log module from Deno standard library

B Use console.log with manual prefixes like [INFO], [ERROR]

C Use Deno.writeTextFile to append logs manually

D Use Deno.inspect to format logs

💡 Hint

Look for a standard library module designed for logging with levels.

Deno's std/log module provides structured logging with levels like info, warning, error, and supports handlers and formatters.

❓ Troubleshoot

advanced

Why does this Deno script fail to catch the error?

This Deno script tries to catch errors from an async function but fails. Why? ```typescript async function readFile() { Deno.readTextFile("nofile.txt"); } try { await readFile(); } catch (e) { console.error("Caught error:", e.message); } ```

A Because try-catch cannot catch errors from async functions

B Because the file actually exists, so no error occurs

C Because Deno.readTextFile is not awaited inside readFile, so error is unhandled

D Because console.error is not called correctly

💡 Hint

Check if the promise returned by Deno.readTextFile is awaited inside the async function.

Without awaiting Deno.readTextFile, the error happens asynchronously and is not caught by the outer try-catch.

🔀 Workflow

advanced

What is the correct order to set up logging with std/log in Deno?

Arrange these steps in the correct order to initialize and use the std/log module for logging in Deno.

A 2,1,3,4

B 1,2,3,4

C 1,3,2,4

D 3,1,2,4

💡 Hint

Think about importing first, then configuring, then initializing, then using.

You must import the module first, then configure the logger, then call setup to initialize, and finally use the logging functions.

✅ Best Practice

expert

Which approach is best for error logging in a Deno production app?

You want to log errors in a Deno app running in production. Which approach is best to ensure errors are logged with context and can be monitored externally?

A Use std/log with custom handlers to send logs to a remote server

B Write errors to a local file using Deno.writeTextFileSync

C Ignore errors to avoid cluttering logs

D Use console.error directly everywhere errors occur

💡 Hint

Think about scalable, centralized logging for production environments.

Using std/log with custom handlers allows sending structured logs to remote monitoring services, which is best practice for production.

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