Deno REPL for quick testing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Deno REPL?

The Deno REPL is an interactive command-line tool where you can type and run Deno code instantly, making it great for quick testing and learning.

beginner

How do you start the Deno REPL?

You start the Deno REPL by running the command deno in your terminal without any file arguments.

intermediate

Can you use modern JavaScript and TypeScript features in the Deno REPL?

Yes, the Deno REPL supports modern JavaScript and TypeScript features, so you can test code using the latest language syntax.

beginner

What happens if you type an expression in the Deno REPL?

The REPL evaluates the expression and immediately shows the result below, helping you see what your code does right away.

beginner

How can the Deno REPL help you during development?

It lets you quickly try out ideas, test small code snippets, and debug without creating files or running full scripts.

How do you launch the Deno REPL?

A Run <code>deno</code> with no arguments

B Run <code>deno run</code> with a file

C Run <code>deno repl start</code>

D Run <code>deno test</code>

The Deno REPL starts by running deno without any file or command arguments.

Which language features can you use in the Deno REPL?

A Only TypeScript without JavaScript

B Only JavaScript ES5

C Modern JavaScript and TypeScript

D Only plain JavaScript without modules

Deno REPL supports modern JavaScript and TypeScript features, allowing flexible testing.

What is the main benefit of using the Deno REPL?

A To write full applications

B To quickly test small code snippets interactively

C To compile code into executables

D To manage Deno permissions

The REPL is designed for quick, interactive testing of code snippets.

What will the Deno REPL do if you enter 2 + 2?

A Show an error

B Ignore the input

C Ask for more input

D Print <code>4</code>

The REPL evaluates expressions and prints the result immediately.

Can you import modules inside the Deno REPL?

A Yes, using <code>import</code> statements

B Only built-in modules can be imported

C Only local files can be imported

D No, imports are not allowed

You can import modules in the REPL using standard import syntax.

Explain how to start and use the Deno REPL for quick testing.

Think about how you open the REPL and what you can do inside it.

Describe the benefits of using the Deno REPL during development.

Consider why an interactive tool helps programmers.

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