Deno REPL for quick testing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the Deno REPL.

Deno

deno [1]

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

Arepl

Brun

Cstart

Dtest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'run' instead of 'repl' to start the REPL.

Trying 'start' which is not a Deno command.

Using 'test' which runs tests, not the REPL.

Use deno repl to start the interactive REPL for quick testing.

2fill in blank

medium

Complete the code to import a function inside the Deno REPL.

Deno

import [1] from "https://deno.land/std@0.177.0/fs/mod.ts";

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

Apath

BreadFileStr

Cfs

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'fs' which is a module, not a function.

Using 'readFileStr' which is for reading files, not checking existence.

Using 'path' which is unrelated here.

Use exists to import the function that checks if a file exists from the Deno standard library.

3fill in blank

hard

Fix the error in the Deno command to allow network access.

Deno

deno run [1] script.ts

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

A--allow-write

B--allow-net

C--allow-env

D--allow-read

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--allow-read' which is for file system read access.

Using '--allow-env' which is for environment variables.

Using '--allow-write' which is for file system write access.

The --allow-net flag grants network access needed by the script.

4fill in blank

hard

Fill both blanks to create a simple Deno command that runs a script with permissions.

Deno

deno run [1] [2] app.ts

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

A--allow-read

B--allow-net

C--allow-env

D--allow-write

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one permission flag.

Using unrelated flags like '--allow-env' or '--allow-write'.

Use --allow-read and --allow-net to allow file reading and network access.

5fill in blank

hard

Fill all three blanks to write a Deno command that runs a script with read, write, and network permissions.

Deno

deno run [1] [2] [3] server.ts

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

A--allow-read

B--allow-net

C--allow-write

D--allow-env

Attempts:

3 left

💡 Hint

Common Mistakes

Missing one or more permission flags.

Using '--allow-env' which is not needed here.

Use --allow-read, --allow-write, and --allow-net to grant read, write, and network permissions respectively.

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