deno fmt for code formatting - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does the command deno fmt do?

Choose the best description of what deno fmt accomplishes when run in a Deno project directory.

A It automatically formats all supported source files in the directory according to Deno's style rules.

B It compiles the source files into executable binaries.

C It runs the tests defined in the project files.

D It installs all dependencies listed in the project.

💡 Hint

Think about what 'fmt' usually stands for in programming tools.

The deno fmt command formats source code files to follow consistent style rules automatically. It does not compile, test, or install dependencies.

❓ component_behavior

intermediate

What happens if you run deno fmt on a file with incorrect indentation?

Given a TypeScript file with inconsistent indentation, what will deno fmt do when executed on that file?

A It will delete the file because of formatting issues.

B It will report an error and refuse to format the file.

C It will fix the indentation and save the file with consistent formatting.

D It will leave the file unchanged.

💡 Hint

Consider the purpose of a formatter tool.

deno fmt automatically corrects formatting issues like indentation and saves the file with the fixed style.

📝 Syntax

advanced

Which command formats only specific files using deno fmt?

You want to format only app.ts and utils.ts files in your project. Which command will do this correctly?

A deno fmt app.ts utils.ts

B deno fmt --files app.ts utils.ts

C deno fmt --only app.ts utils.ts

D deno fmt -f app.ts utils.ts

💡 Hint

Check the official syntax for specifying files with deno fmt.

The deno fmt command accepts file paths directly as arguments to format only those files.

🔧 Debug

advanced

Why does deno fmt not format a file?

You run deno fmt myfile.ts but the file remains unchanged even though it has formatting issues. What is the most likely reason?

A The file has syntax errors preventing formatting.

B The file is already formatted according to Deno's style rules.

C The <code>deno fmt</code> command was run without permissions.

D The file is ignored by a configuration file.

💡 Hint

Think about what happens if a formatter sees a file already correct.

If the file matches Deno's formatting rules, deno fmt will not change it.

❓ state_output

expert

What is the output of running deno fmt --check on a badly formatted file?

You run deno fmt --check bad.ts where bad.ts has inconsistent spacing and indentation. What will be the output?

A It throws a syntax error and stops.

B It reformats the file automatically and prints 'Formatted successfully'.

C It ignores the file and prints nothing.

D It prints the names of files that need formatting and exits with a non-zero status.

💡 Hint

The --check flag is for verifying formatting without changing files.

The --check option reports files that are not formatted correctly and exits with an error code, without changing files.

Practice

1. What is the main purpose of the deno fmt command?

easy

A. To install third-party modules

B. To run your Deno program

C. To automatically format your code for better readability and consistency

D. To check for security vulnerabilities in your code

Solution

1. Step 1: Understand the role of deno fmt

   deno fmt is designed to format code automatically, making it easier to read and consistent across files.

2. Step 2: Differentiate from other commands

   Running programs, installing modules, or security checks are done by other commands like deno run, deno install, or deno lint.

3. Final Answer:

   To automatically format your code for better readability and consistency -> Option C

4. Quick Check:

   deno fmt formats code [OK]

Hint: Remember: fmt means format code automatically [OK]

Common Mistakes:

• Confusing fmt with run or install commands
• Thinking fmt checks code errors instead of formatting
• Assuming fmt modifies program logic

2. Which of the following is the correct way to run deno fmt on all supported files in your project?

easy

A. deno fmt

B. deno fmt install

C. deno fmt run

D. deno fmt --check

Solution

1. Step 1: Identify the basic command to format files

   Running deno fmt without extra flags formats all supported files in the current directory and subdirectories.

2. Step 2: Understand other options

   --check only checks formatting without changing files; run and install are unrelated commands.

3. Final Answer:

   deno fmt -> Option A

4. Quick Check:

   Basic command to format = deno fmt [OK]

Hint: Use just 'deno fmt' to format all files quickly [OK]

Common Mistakes:

• Adding unnecessary flags like run or install
• Using --check when wanting to format files
• Confusing fmt with other Deno commands

3. Given this unformatted TypeScript code in example.ts:

function greet ( name:string ){console.log( "Hello, " +name );}

What will be the output in example.ts after running deno fmt?

medium

A. function greet ( name:string ){console.log( "Hello, " +name );}

B. function greet(name: string) {
console.log("Hello, " + name);
}

C. function greet(name:string){console.log("Hello,"+name);}

D. function greet(name: string) {console.log("Hello," + name);}

Solution

1. Step 1: Understand what deno fmt does to code style

   deno fmt adds spaces after commas, spaces around operators, and proper indentation and line breaks for readability.

2. Step 2: Compare options with expected formatting

   function greet(name: string) { console.log("Hello, " + name); } shows proper spacing after parameters, braces on new lines, and indentation inside the function, matching deno fmt style.

3. Final Answer:

   function greet(name: string) { console.log("Hello, " + name); } -> Option B

4. Quick Check:

   deno fmt adds spaces and indentation [OK]

Hint: Look for spaces after colons and proper indentation [OK]

Common Mistakes:

• Ignoring spaces after colons and commas
• Not indenting inside function blocks
• Keeping all code on one line

4. You run deno fmt --check on your project and get an error saying some files are not formatted. What should you do to fix this?

medium

A. Run deno fmt without flags to format the files automatically

B. Ignore the error because it does not affect code execution

C. Delete the files that are not formatted

D. Run deno run to fix formatting issues

Solution

1. Step 1: Understand what deno fmt --check does

   This command checks if files are formatted but does not change them.

2. Step 2: Fix formatting issues

   To fix formatting, run deno fmt without flags to automatically format all files.

3. Final Answer:

   Run deno fmt without flags to format the files automatically -> Option A

4. Quick Check:

   Use deno fmt to fix formatting [OK]

Hint: Use deno fmt (no flags) to fix formatting errors [OK]

Common Mistakes:

• Ignoring formatting errors thinking they don't matter
• Trying to fix formatting by running code
• Deleting files instead of formatting

5. You want to ensure your project code is always formatted before committing to Git. Which command can you add to your Git pre-commit hook to automatically format all files using Deno?

hard

A. deno run

B. deno install

C. deno lint

D. deno fmt

Solution

1. Step 1: Identify the command that formats code automatically

   deno fmt formats all supported files, making it ideal for pre-commit hooks to keep code clean.

2. Step 2: Understand other commands' purposes

   deno run executes scripts, deno lint checks code style but does not fix formatting, and deno install installs scripts.

3. Final Answer:

   deno fmt -> Option D

4. Quick Check:

   Pre-commit formatting uses deno fmt [OK]

Hint: Use deno fmt in pre-commit hooks to auto-format code [OK]

Common Mistakes:

• Using deno run or deno lint instead of fmt
• Thinking lint fixes formatting automatically
• Not automating formatting before commits