Running your first Deno program

Introduction

Deno lets you run JavaScript and TypeScript code easily on your computer. Running your first program shows you how to start using Deno.

When you want to try out Deno for the first time.

When you want to run a simple script to see how Deno works.

When you want to check if Deno is installed correctly on your computer.

Syntax

Deno

deno run filename.ts

Replace filename.ts with your actual file name.

Deno supports both JavaScript (.js) and TypeScript (.ts) files.

Examples

This runs a TypeScript file named hello.ts.

Deno

deno run hello.ts

This runs a JavaScript file named script.js.

Deno

deno run script.js

Sample Program

This simple program prints a greeting message to the console. Save it as hello.ts and run it with deno run hello.ts.

Deno

console.log("Hello, Deno!");

Important Notes

Deno requires explicit permission for some actions like reading files or accessing the network. For simple console logs, no extra permissions are needed.

Use the terminal or command prompt to run Deno commands.

Summary

Deno runs JavaScript and TypeScript files using the deno run command.

Start with a simple program that prints text to the console.

Make sure your file name matches what you type after deno run.

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