Error handling and logging in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch errors using try-catch.

Deno

try {
  const data = await fetch('https://api.example.com/data');
  console.log(await data.json());
} catch ([1]) {
  console.error('Error fetching data:', error);
}

Drag options to blanks, or click blank then click option'

Aerr

Berror

Cexception

De

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name in catch that doesn't match the one used inside the block.

Forgetting to use try-catch around async code.

The catch block uses error as the variable to hold the caught error.

2fill in blank

medium

Complete the code to log an info message using Deno's standard logger.

Deno

import { log } from "https://deno.land/std/log/mod.ts";

await log.setup({
  handlers: {
    console: new log.handlers.ConsoleHandler("INFO"),
  },
  loggers: {
    default: {
      level: "INFO",
      handlers: ["console"],
    },
  },
});

log.[1]("This is an info message.");

Drag options to blanks, or click blank then click option'

Awarning

Berror

Cdebug

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using a log level method that does not match the setup level.

Forgetting to call await on log.setup before logging.

The info method logs informational messages.

3fill in blank

hard

Fix the error in the async function to properly handle errors.

Deno

async function getData() {
  try {
    const response = await fetch('https://api.example.com/data');
    if (!response.ok) throw new Error('Network response was not ok');
    const data = await response.json();
    return data;
  } catch ([1]) {
    console.log('Fetch failed:', error.message);
  }
}

Drag options to blanks, or click blank then click option'

Aerror

Cerr

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses in catch block.

Using a variable name without parentheses.

The catch block must have parentheses around the error variable, like catch (error).

4fill in blank

hard

Fill both blanks to create a logger that writes error messages to a file.

Deno

import { log } from "https://deno.land/std/log/mod.ts";

await log.setup({
  handlers: {
    file: new log.handlers.[1]("ERROR", {
      filename: "error.log",
      formatter: log.formatters.[2](),
    }),
  },
  loggers: {
    default: {
      level: "ERROR",
      handlers: ["file"],
    },
  },
});

Drag options to blanks, or click blank then click option'

AFileHandler

BConsoleHandler

Cjson

Dsimple

Attempts:

3 left

💡 Hint

Common Mistakes

Using ConsoleHandler instead of FileHandler for file logging.

Using 'json' formatter when 'simple' is expected.

Use FileHandler to write logs to a file and simple as a common formatter.

5fill in blank

hard

Fill all three blanks to create a custom error class and throw it with a message.

Deno

class [1] extends Error {
  constructor(message) {
    super(message);
    this.name = "[2]";
  }
}

throw new [3]("Custom error occurred");

Drag options to blanks, or click blank then click option'

ACustomError

BError

DMyError

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatch between class name and error name string.

Throwing a different error class than the one defined.

The class name and error name should match. The thrown error is an instance of the custom class.

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