deno lint for code quality - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the lint error in this Deno code snippet

What lint error will deno lint report for the following code?

Deno

const unusedVar = 42;
console.log("Hello Deno!");

A Error: 'unusedVar' is declared but its value is never read

B No errors, code is clean

C Error: Missing semicolon at the end of line 2

D Error: console.log is not allowed

💡 Hint

Think about variables declared but never used in the code.

Deno lint checks for unused variables and will report 'unusedVar' as unused because it is declared but never used.

❓ component_behavior

intermediate

Effect of linting on Deno project files

After running deno lint on a project, what is the expected behavior?

A It reports lint errors and warnings without changing files

B It automatically fixes all lint errors in the code

C It deletes files with lint errors

D It compiles the code to JavaScript

💡 Hint

Consider if deno lint modifies your source files or just reports issues.

deno lint only reports lint errors and warnings. It does not modify or fix files automatically.

🔧 Debug

advanced

Diagnose the lint error from this Deno code

What specific lint error will deno lint report for this code?

Deno

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

A No lint errors

B Error: Missing argument in function call to 'greet'

C Error: Function 'greet' is declared but never used

D Error: 'name' is possibly undefined

💡 Hint

Check the function parameter usage and how it is called.

The function expects a string argument, but it is called without one. Deno lint reports an error for the missing argument in the function call.

❓ state_output

advanced

Output of deno lint with a style violation

What output will deno lint produce for this code snippet?

Deno

const message = "Hello Deno"
console.log(message)

A No errors, code is clean

B Error: 'message' is declared but never used

C Error: Missing semicolon at the end of line 1

D Warning: Missing semicolon at the end of line 1

💡 Hint

Think about style rules like semicolons and how Deno lint reports them.

Deno lint includes the 'semi' rule (tagged 'recommended'), which requires semicolons and reports missing semicolons as errors.

🧠 Conceptual

expert

Understanding deno lint configuration impact

Given a deno.json configuration with "lint": { "rules": { "tags": ["recommended"] } }, what effect does this have on deno lint?

A Linting will be disabled completely

B Lint will automatically fix all issues tagged 'recommended'

C All lint rules except those tagged 'recommended' will be checked

D Only lint rules tagged as 'recommended' will be checked

💡 Hint

Consider how specifying tags in lint rules filters which rules run.

Specifying tags: ["recommended"] means only lint rules with the 'recommended' tag are applied during linting.

Practice

1. What is the main purpose of running deno lint on your code?

easy

A. To execute the code and show output

B. To find and report code style and potential error issues

C. To compile the code into a binary

D. To install external dependencies automatically

Solution

1. Step 1: Understand what linting means

   Linting is a process that checks code for style and possible mistakes without running it.

2. Step 2: Identify what deno lint does

   deno lint scans your code files and reports style issues or errors to improve code quality.

3. Final Answer:

   To find and report code style and potential error issues -> Option B

4. Quick Check:

   Linting = Find code issues [OK]

Hint: Lint checks code style and errors, not execution or compilation [OK]

Common Mistakes:

• Confusing lint with running or compiling code
• Thinking lint installs dependencies
• Assuming lint fixes all errors automatically

2. Which command correctly runs the linter on all files in the current folder and subfolders?

easy

A. deno lint --fix

B. deno run lint

C. deno lint .

D. deno lint --all

Solution

1. Step 1: Identify the command to lint files

   The command deno lint followed by a path runs lint on that path.

2. Step 2: Understand the meaning of '.' in commands

   Using '.' means current directory and all files inside it recursively.

3. Final Answer:

   deno lint . -> Option C

4. Quick Check:

   Lint current folder = deno lint . [OK]

Hint: Use dot (.) to lint current folder and subfolders [OK]

Common Mistakes:

• Using 'deno run lint' which is invalid
• Assuming '--all' is a valid flag for deno lint
• Confusing '--fix' as a way to run lint only

3. Given this code snippet in example.ts:

const unusedVar = 5;
console.log("Hello Deno!");

What will deno lint example.ts report?

medium

A. Error: unused variable 'unusedVar'

B. Warning: missing semicolon

C. Syntax error on line 1

D. No issues found

Solution

1. Step 1: Analyze the code for lint issues

   The variable unusedVar is declared but never used, which is a common lint warning.

2. Step 2: Understand what deno lint reports

   Deno lint reports unused variables as errors or warnings to improve code quality.

3. Final Answer:

   Error: unused variable 'unusedVar' -> Option A

4. Quick Check:

   Unused variable = lint error [OK]

Hint: Unused variables cause lint errors, check variable usage [OK]

Common Mistakes:

• Thinking no issues appear if code runs
• Confusing lint errors with syntax errors
• Expecting semicolon warnings when not configured

4. You run deno lint and get errors about unused imports. Which fix will resolve this?

medium

A. Ignore the errors and continue running the code

B. Add --fix flag to automatically remove unused imports

C. Rename the imports to different variable names

D. Remove the unused import lines from the code

Solution

1. Step 1: Understand what unused import errors mean

   Unused imports are lines importing modules or variables that are never used in the code.

2. Step 2: Identify the correct fix

   Removing those unused import lines cleans the code and fixes the lint errors. The --fix flag does not always remove imports automatically.

3. Final Answer:

   Remove the unused import lines from the code -> Option D

4. Quick Check:

   Remove unused imports to fix lint errors [OK]

Hint: Delete unused imports manually; --fix may not remove them [OK]

Common Mistakes:

• Assuming --fix always removes unused imports
• Ignoring lint errors thinking code still runs
• Renaming imports without using them

5. You want to automatically fix some lint issues in your project folder. Which command should you run?

hard

A. deno lint --fix ./

B. deno lint --auto-fix ./

C. deno lint fix ./

D. deno lint --correct ./

Solution

1. Step 1: Recall the flag for automatic fixing in deno lint

   The correct flag to fix some lint issues automatically is --fix.

2. Step 2: Confirm the command syntax

   The command deno lint --fix ./ runs lint on the current folder and applies automatic fixes where possible.

3. Final Answer:

   deno lint --fix ./ -> Option A

4. Quick Check:

   Use --fix flag to auto-correct lint issues [OK]

Hint: Use --fix flag with deno lint to auto-correct issues [OK]

Common Mistakes:

• Using incorrect flags like --auto-fix or --correct
• Typing 'deno lint fix' without dashes
• Not specifying the folder or file to lint