TypeScript support out of the box in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a TypeScript file directly with Deno.

Deno

deno run [1]

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

Aapp.ts

Bapp.json

Capp.js

Dapp.py

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to run JavaScript or other file types instead of TypeScript.

Deno runs TypeScript files directly without extra setup. Use the .ts file extension.

2fill in blank

medium

Complete the code to import a TypeScript module in Deno.

Deno

import [1] from './mod.ts';

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

Atrue

BmyModule

C123

Dmodule

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid variable names like numbers or reserved words.

You import a module by giving it a valid variable name to use in your code.

3fill in blank

hard

Fix the error in the TypeScript code to declare a variable with a string type.

Deno

let name: [1] = 'Deno';

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

Aobject

Bnumber

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean types for string values.

The variable name holds text, so its type should be string.

4fill in blank

hard

Fill both blanks to define a function with typed parameters and return type in TypeScript.

Deno

function greet(name: [1]): [2] { return `Hello, ${name}`; }

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

Astring

Bnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or void as return type when the function returns text.

The parameter name is a string, and the function returns a string greeting.

5fill in blank

hard

Fill all three blanks to create a typed object and access its property in TypeScript.

Deno

const user: [1] = { name: [2] };
console.log(user.[3]);

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

A{ name: string }

B'Alice'

Cname

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong type syntax or accessing a non-existent property.

The object user has a type with a name string property. We assign the string 'Alice' and access the name property.

Practice

1. What is a key feature of Deno regarding TypeScript support?

easy

A. Deno does not support TypeScript at all.

B. Deno requires manual compilation of TypeScript before running.

C. Deno runs TypeScript files directly without extra setup.

D. Deno only supports JavaScript, not TypeScript.

Solution

1. Step 1: Understand Deno's TypeScript handling

   Deno is designed to run TypeScript files directly without needing a separate compilation step.

2. Step 2: Compare options with this feature

   Only Deno runs TypeScript files directly without extra setup. correctly states this direct support; others contradict this fact.

3. Final Answer:

   Deno runs TypeScript files directly without extra setup. -> Option C

4. Quick Check:

   Direct TypeScript execution = A [OK]

Hint: Remember: Deno runs TS files without compiling first [OK]

Common Mistakes:

• Thinking Deno needs manual TypeScript compilation
• Believing Deno only supports JavaScript
• Confusing Deno with Node.js setup

2. Which of the following is the correct way to run a TypeScript file named app.ts using Deno?

easy

A. deno compile app.ts

B. tsc app.ts && deno run app.js

C. node app.ts

D. deno run app.ts

Solution

1. Step 1: Identify Deno command to run TS files

   Deno uses deno run to execute TypeScript files directly.

2. Step 2: Evaluate other options

   deno compile app.ts compiles but does not run; tsc app.ts && deno run app.js compiles manually then runs JS, which is unnecessary; node app.ts uses Node.js which can't run TS directly.

3. Final Answer:

   deno run app.ts -> Option D

4. Quick Check:

   Run TS file with 'deno run' = C [OK]

Hint: Use 'deno run' to execute TypeScript files directly [OK]

Common Mistakes:

• Trying to compile TS manually before running
• Using Node.js command for TS files
• Confusing compile and run commands

3. Given this TypeScript code run with Deno:

function greet(name: string) {
  return `Hello, ${name}!`;
}
console.log(greet(42));

What will happen when you run deno run on this file?

medium

A. It throws a type error before running

B. It prints 'Hello, 42!'

C. It prints 'Hello, undefined!'

D. It runs but logs 'Hello, 42!' with a warning

Solution

1. Step 1: Analyze TypeScript type checking

   The function expects a string, but 42 is a number, causing a type error.

2. Step 2: Understand Deno's behavior

   Deno checks types before running and will throw an error, stopping execution.

3. Final Answer:

   It throws a type error before running -> Option A

4. Quick Check:

   Type mismatch causes error = A [OK]

Hint: Type errors stop Deno before running code [OK]

Common Mistakes:

• Assuming Deno runs code ignoring type errors
• Thinking it prints with warnings
• Confusing runtime errors with type errors

4. You wrote this TypeScript code:

const add = (a: number, b: number) => a + b;
console.log(add('5', 10));

When running with Deno, you get an error. What is the best fix?

medium

A. Remove type annotations from the function.

B. Change '5' to 5 (number) in the function call.

C. Run with Node.js instead of Deno.

D. Ignore the error and run anyway.

Solution

1. Step 1: Identify the type mismatch

   The function expects numbers, but '5' is a string, causing a type error.

2. Step 2: Fix the argument type

   Changing '5' (string) to 5 (number) matches the expected types and fixes the error.

3. Final Answer:

   Change '5' to 5 (number) in the function call. -> Option B

4. Quick Check:

   Correct argument types fix errors = B [OK]

Hint: Match argument types to function parameters [OK]

Common Mistakes:

• Removing types loses safety
• Trying to run with Node.js without compiling
• Ignoring errors instead of fixing types

5. You want to write a Deno script that reads a JSON file and ensures the data matches a TypeScript interface before processing. Which approach best uses Deno's TypeScript support out of the box?

hard

A. Define the interface, read the file, parse JSON, and use a type assertion to check the data.

B. Write JavaScript code and manually check types at runtime with if statements.

C. Compile the TypeScript to JavaScript first, then run with Deno.

D. Use a third-party tool to convert JSON to TypeScript types before running.

Solution

1. Step 1: Use TypeScript interface for type safety

   Defining an interface helps TypeScript check data structure during development.

2. Step 2: Read and parse JSON, then assert type

   Parsing JSON and using a type assertion leverages Deno's built-in TypeScript support to catch errors early.

3. Final Answer:

   Define the interface, read the file, parse JSON, and use a type assertion to check the data. -> Option A

4. Quick Check:

   Use interfaces and assertions for type safety = D [OK]

Hint: Use interfaces and type assertions with Deno's TS support [OK]

Common Mistakes:

• Ignoring TypeScript interfaces and relying on runtime checks
• Compiling manually before running with Deno
• Overcomplicating with external tools unnecessarily