deno lint for code quality - Mini Project: Build & Apply

Using deno lint for Code Quality

📖 Scenario: You are working on a small Deno project that includes a simple script. You want to ensure your code follows good style and quality rules before sharing it with others.

🎯 Goal: Learn how to set up and run deno lint to check your Deno script for code quality issues.

📋 What You'll Learn

Create a simple Deno script file named app.ts with a function and a call

Add a configuration file deno.json to customize lint rules

Run deno lint to check the script for linting errors

Fix a lint warning by updating the code

💡 Why This Matters

🌍 Real World

Using <code>deno lint</code> helps keep your Deno code clean and consistent, which is important when working on projects with others or preparing code for production.

💼 Career

Many software development jobs require writing clean, maintainable code. Knowing how to use linting tools like <code>deno lint</code> is a valuable skill for ensuring code quality and following team coding standards.

1

Create a simple Deno script

Create a file named app.ts and write a function called greet that takes a name parameter and returns the string `Hello, ${name}!`. Then call greet with the argument "World" and store the result in a variable called message.

Deno

# Your code here

Hint

Define a function with function greet(name: string): string and use a template string to return the greeting.

2

Add a deno.json configuration file

Create a file named deno.json and add a lint section with rules that disables the no-explicit-any rule by setting it to false.

Deno

function greet(name: string): string {
  return `Hello, ${name}!`;
}

const message = greet("World");

// Your code here to create deno.json

Hint

The deno.json file is a JSON file where you add a lint object with rules inside.

3

Run deno lint on the script

Run the command deno lint app.ts in your terminal to check the app.ts file for linting errors.

Deno

function greet(name: string): string {
  return `Hello, ${name}!`;
}

const message = greet("World");

// Run deno lint app.ts in your terminal
// Your code here

Hint

Use your terminal or command prompt to run deno lint app.ts.

4

Fix a lint warning by updating the code

If deno lint reports a warning about an unused variable, update the code by adding console.log(message); after the message variable declaration to use it.

Deno

function greet(name: string): string {
  return `Hello, ${name}!`;
}

const message = greet("World");

// Add console.log(message); below to fix unused variable warning
// Your code here

Hint

Use console.log(message); to use the variable and remove the lint warning.

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