Deno REPL for quick testing

Introduction

Deno REPL lets you quickly try out JavaScript or TypeScript code without making files. It helps you test ideas fast.

You want to check how a small piece of code works before adding it to your project.

You need to experiment with new JavaScript or TypeScript features quickly.

You want to debug or explore Deno APIs interactively.

You want to learn Deno commands and syntax by trying examples live.

Syntax

Deno

deno repl

Run this command in your terminal to start the REPL (Read-Eval-Print Loop).

You can type JavaScript or TypeScript code directly and see results immediately.

Examples

Starts the Deno REPL where you can type code interactively.

Deno

deno repl

Prints a message and shows the result of the console.log call.

Deno

> console.log('Hello from Deno REPL!')
Hello from Deno REPL!
undefined

Defines a function and then calls it to get the sum.

Deno

> const sum = (a, b) => a + b
undefined
> sum(5, 7)
12

Sample Program

This example shows starting the REPL, defining a simple function, and calling it to see the output immediately.

Deno

deno repl
// Then type the following lines inside the REPL:
> const greet = (name) => `Hello, ${name}!`
undefined
> greet('friend')
'Hello, friend!'

Important Notes

The REPL supports both JavaScript and TypeScript syntax.

Use Ctrl+C twice to exit the REPL safely.

You can import modules inside the REPL using import statements.

Summary

Deno REPL is a quick way to test code without files.

It helps you learn and debug interactively.

Start it by running deno repl in your terminal.

Practice

1. What is the main purpose of the deno repl command?

easy

A. To install Deno packages from the internet

B. To start an interactive environment for testing Deno code quickly

C. To compile Deno scripts into executable files

D. To create a new Deno project folder

Solution

1. Step 1: Understand what REPL means

   REPL stands for Read-Eval-Print Loop, an interactive shell to run code instantly.

2. Step 2: Match REPL purpose with Deno command

   The deno repl command opens this interactive shell for Deno code testing.

3. Final Answer:

   To start an interactive environment for testing Deno code quickly -> Option B

4. Quick Check:

   Deno REPL = quick interactive testing [OK]

Hint: REPL means interactive testing shell in Deno [OK]

Common Mistakes:

• Confusing REPL with compiling scripts
• Thinking REPL installs packages
• Assuming REPL creates project folders

2. Which of the following is the correct way to start the Deno REPL in your terminal?

easy

A. deno run repl

B. deno shell

C. deno start repl

D. deno repl

Solution

1. Step 1: Recall the exact command to start Deno REPL

   The official command is deno repl to launch the interactive shell.

2. Step 2: Check other options for correctness

   Options like deno run repl, deno start repl, and deno shell are invalid commands.

3. Final Answer:

   deno repl -> Option D

4. Quick Check:

   Start Deno REPL = deno repl [OK]

Hint: Use exactly 'deno repl' to start the REPL [OK]

Common Mistakes:

• Adding extra words like 'run' or 'start'
• Using 'deno shell' which is not a Deno command
• Confusing with Node.js REPL commands

3. What will be the output when you run the following code inside the Deno REPL?

const x = 5;
const y = 10;
x + y;

medium

A. 15

B. undefined

C. NaN

D. SyntaxError

Solution

1. Step 1: Evaluate the code line by line in REPL context

   Variables x and y are assigned 5 and 10 respectively.

2. Step 2: Calculate the expression x + y

   Adding 5 + 10 results in 15, which REPL will print as output.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: REPL prints last expression value automatically [OK]

Common Mistakes:

• Expecting no output or undefined
• Confusing with syntax errors
• Thinking variables are not stored in REPL

4. You typed the following in the Deno REPL but got an error:

let a = 10
let b = 20
console.log(a + b)

What is the likely cause of the error?

medium

A. Missing semicolons causing syntax error in REPL

B. Deno REPL does not support console.log

C. Variables must be declared with var, not let

D. You need to run the code in a file, not REPL

Solution

1. Step 1: Check syntax rules in Deno REPL

   Deno REPL requires semicolons or line breaks to separate statements properly.

2. Step 2: Identify missing semicolons causing error

   Missing semicolons after let a = 10 and let b = 20 cause syntax error in REPL.

3. Final Answer:

   Missing semicolons causing syntax error in REPL -> Option A

4. Quick Check:

   Semicolons needed in REPL lines [OK]

Hint: Add semicolons or press Enter twice in REPL [OK]

Common Mistakes:

• Assuming console.log is unsupported
• Thinking let is invalid in REPL
• Believing REPL can't run multi-line code

5. You want to quickly test a function that returns the square of a number in Deno REPL. Which sequence of commands will correctly define and test it?

hard

A. function square(n) {
return n * n
}
console.log(square(4));

B. function square(n) {
return n * n
}
square(4)

C. function square(n) {
return n * n;
}
square(4);

D. function square(n) {
return n * n;
}
console.log(square(4))

Solution

1. Step 1: Define the function with proper syntax and semicolons

   function square(n) {
     return n * n;
   }
   square(4);

   uses semicolons and braces correctly, which is good practice in REPL.

2. Step 2: Call the function as an expression to see output

   Calling square(4); as last line prints 16 automatically in REPL.

3. Final Answer:

   <pre>function square(n) { return n * n; } square(4);</pre> -> Option C

4. Quick Check:

   Function + call + semicolons = correct REPL test [OK]

Hint: End lines with semicolons and call function last [OK]

Common Mistakes:

• Omitting semicolons causing REPL errors
• Using console.log which prints undefined in REPL
• Not calling the function as last expression