Running your first Deno program - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Deno?

Deno is a modern runtime for JavaScript and TypeScript that runs code outside the browser. It is secure by default and easy to use.

beginner

How do you run a simple Deno program named hello.ts?

Use the command deno run hello.ts in your terminal to execute the program.

intermediate

Why does Deno require explicit permissions to access files or network?

Deno is secure by default. It asks for permissions to protect your computer from harmful code accessing files, network, or environment without your approval.

intermediate

What command would you use to run a Deno program that needs to read a file?

You run deno run --allow-read your_program.ts to give permission for reading files.

intermediate

What will happen if you try to run a Deno program that accesses the network without permission?

Deno will stop the program and show an error because it blocks network access unless you allow it with --allow-net.

Which command runs a Deno program named app.ts?

A npm run app.ts

B node app.ts

C deno start app.ts

D deno run app.ts

Deno programs run with the command 'deno run filename'. Node and npm commands do not run Deno programs.

What flag do you add to allow a Deno program to read files?

A --allow-net

B --allow-read

C --allow-write

D --allow-env

The flag '--allow-read' gives permission to read files. '--allow-net' is for network access.

If you run a Deno program without any permission flags, what happens when it tries to access the network?

A It crashes with an error.

B It ignores the network request.

C It asks for permission interactively.

D It works fine.

Deno blocks network access without permission and throws an error.

Which of these is NOT a feature of Deno?

A Requires a package.json file

B Supports TypeScript out of the box

C Secure by default

D Runs JavaScript outside the browser

Deno does not require a package.json file; it uses URL imports and modules.

What is the first step to run your first Deno program?

A Install Node.js

B Create a package.json file

C Write your code in a file with .ts or .js extension

D Run npm install

You start by writing your code in a file. Node.js and npm are not needed for Deno.

Explain how to run a simple Deno program from writing the code to executing it.

Think about the steps from code to running in terminal.

Describe why Deno requires permission flags and give examples of common flags.

Consider security and how Deno protects your computer.

Practice

1. What command do you use to run a JavaScript or TypeScript file with Deno?

easy

A. node filename.js

B. deno run filename.ts

C. npm start filename.ts

D. deno start filename.js

Solution

1. Step 1: Understand the Deno run command

   Deno uses the command deno run followed by the file name to execute scripts.

2. Step 2: Identify the correct command for running a file

   The correct syntax is deno run filename.ts or deno run filename.js. Other options are for Node.js or incorrect commands.

3. Final Answer:

   deno run filename.ts -> Option B

4. Quick Check:

   Deno runs scripts with 'deno run' [OK]

Hint: Use 'deno run' plus your file name to run scripts [OK]

Common Mistakes:

• Using 'node' instead of 'deno run'
• Typing 'deno start' which is invalid
• Using 'npm start' which is for Node.js projects

2. Which of the following is the correct syntax to print 'Hello, Deno!' to the console in a Deno program?

easy

A. console.print('Hello, Deno!')

B. print('Hello, Deno!')

C. console.log('Hello, Deno!')

D. echo('Hello, Deno!')

Solution

1. Step 1: Recall JavaScript console output method

   In JavaScript and TypeScript, the standard way to print to the console is using console.log().

2. Step 2: Identify the correct function

   Options like print() or echo() are not valid in JavaScript or Deno. console.print() is not a function.

3. Final Answer:

   console.log('Hello, Deno!') -> Option C

4. Quick Check:

   Use console.log() to print text [OK]

Hint: Use console.log() to print messages in Deno [OK]

Common Mistakes:

• Using print() which is not defined in JavaScript
• Trying console.print() which does not exist
• Using echo() which is from shell scripting

3. What will be the output when you run this Deno program?

console.log('Welcome to Deno!');

medium

A. Welcome to Deno!

B. console.log('Welcome to Deno!');

C. SyntaxError

D. No output

Solution

1. Step 1: Understand console.log behavior

   The console.log() function prints the string inside the parentheses to the console.

2. Step 2: Predict the output

   The string 'Welcome to Deno!' will be printed exactly as is, without quotes.

3. Final Answer:

   Welcome to Deno! -> Option A

4. Quick Check:

   console.log prints the string content [OK]

Hint: console.log prints text without quotes [OK]

Common Mistakes:

• Expecting the code line itself as output
• Thinking it causes a syntax error
• Assuming no output without running

4. You wrote this Deno program:

console.log('Hello Deno!')

But when you run deno run hello.ts, you get an error. What is the most likely cause?

medium

A. You must run with 'deno start' instead

B. You need to add a semicolon at the end

C. Deno does not support console.log

D. You forgot to save the file before running

Solution

1. Step 1: Check common causes of errors when running Deno

   If the file is not saved, Deno cannot find the latest code to run, causing errors.

2. Step 2: Evaluate other options

   Semicolons are optional in JavaScript/TypeScript, console.log is supported, and 'deno start' is not a valid command.

3. Final Answer:

   You forgot to save the file before running -> Option D

4. Quick Check:

   Unsaved files cause run errors [OK]

Hint: Always save your file before running with Deno [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Believing console.log is unsupported
• Using wrong Deno commands like 'deno start'

5. You want to run a Deno program named greet.ts that prints a greeting. Which command will correctly run the program if your terminal is in the same folder as the file?

medium

A. deno run ./greet.ts

B. deno start greet.ts

C. deno run ../greet.ts

D. deno rungreet.ts

Solution

1. Step 1: Understand file path usage in Deno run

   Deno requires the file path to be correct. Using ./greet.ts explicitly points to the current directory.

2. Step 2: Evaluate other options

   deno rungreet.ts is invalid because there's no space after run. deno run ../greet.ts points to the parent directory, which is wrong here. deno start greet.ts is invalid.

3. Final Answer:

   deno run ./greet.ts -> Option A

4. Quick Check:

   Use './filename' to run files in current folder [OK]

Hint: Use './filename' to run files in current directory [OK]

Common Mistakes:

• Missing space after 'run' like 'deno rungreet.ts'
• Using '../' which points to wrong folder
• Trying 'deno start' which is invalid