TypeScript support out of the box in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean that Deno has TypeScript support out of the box?

It means Deno can run TypeScript code directly without needing extra setup or compilation steps. You just write TypeScript, and Deno handles the rest.

beginner

How does Deno handle TypeScript files when you run them?

Deno automatically compiles TypeScript files to JavaScript behind the scenes before running them, so you don't have to manually compile.

beginner

Why is having TypeScript support out of the box helpful for beginners?

It removes the need to learn complex build tools or configurations. Beginners can focus on writing code and learning TypeScript directly.

beginner

Can you run JavaScript files in Deno as well as TypeScript files?

Yes, Deno supports both JavaScript and TypeScript files natively, so you can run either without extra setup.

beginner

What command do you use to run a TypeScript file in Deno?

You use the command deno run filename.ts. Deno will handle the TypeScript automatically.

What does Deno do when you run a TypeScript file?

A Requires you to compile it manually first

B Throws an error because TypeScript is not supported

C Runs it as plain JavaScript without compiling

D Automatically compiles it to JavaScript and runs it

Deno automatically compiles TypeScript to JavaScript behind the scenes before running.

Which command runs a TypeScript file in Deno?

A deno run filename.ts

B tsc filename.ts

C node filename.ts

D deno compile filename.ts

The command deno run filename.ts runs TypeScript files directly.

Why is Deno’s TypeScript support called 'out of the box'?

A Because it compiles TypeScript manually

B Because you must install plugins first

C Because it requires no extra setup or tools

D Because it only supports JavaScript

Deno supports TypeScript immediately without extra setup.

Can Deno run JavaScript files without any changes?

A No, it only runs TypeScript

B Yes, it supports JavaScript and TypeScript natively

C Only if you convert JavaScript to TypeScript first

D Only with a special flag

Deno runs both JavaScript and TypeScript files natively.

What benefit does Deno’s TypeScript support provide to new learners?

A They can write TypeScript without worrying about setup

B They must learn complex build tools first

C They can only write JavaScript

D They need to install extra compilers

Deno lets beginners focus on coding TypeScript without setup hassle.

Explain how Deno supports TypeScript out of the box and why this is helpful.

Think about what 'out of the box' means for running code.

Describe the command you use to run a TypeScript file in Deno and what happens behind the scenes.

Focus on the command and the automatic process.

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