Deno REPL for quick testing - Step-by-Step Execution

Concept Flow - Deno REPL for quick testing

Start Deno REPL

↓

Type JavaScript/TypeScript code

↓

Press Enter

↓

Deno evaluates code

↓

Output result shown

↓

Repeat or exit with Ctrl+C twice

You start the Deno REPL, type code, press Enter, see the result, and repeat or exit.

Execution Sample

Deno

deno
> let x = 5;
> x * 2
10

This shows starting Deno REPL, defining a variable, and evaluating an expression.

Execution Table

Step	Input	Action	Output
1	deno	Start Deno REPL	REPL ready for input
2	let x = 5;	Define variable x with value 5	undefined
3	x * 2	Evaluate expression multiplying x by 2	10
4	console.log('Hi')	Print string to console	Hi undefined
5	Ctrl+C twice	Exit REPL	REPL closed

💡 User exits REPL by pressing Ctrl+C twice

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
x	undefined	5	5	5

Key Moments - 3 Insights

Why does defining a variable show 'undefined' as output?

In the REPL, statements like variable declarations do not return a value, so the output is 'undefined' as shown in step 2 of the execution_table.

How does the REPL show the result of expressions?

When you type an expression like 'x * 2', the REPL evaluates it and immediately shows the result, as seen in step 3.

How do you exit the Deno REPL?

You press Ctrl+C twice to exit, which is shown in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after typing 'let x = 5;'?

A "5"

B "undefined"

C "10"

D "REPL ready for input"

💡 Hint

Check step 2 output in the execution_table.

At which step does the REPL show the result of an expression?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for when 'x * 2' is evaluated in the execution_table.

If you type 'console.log("Hi")' in the REPL, what output do you see?

A "Hi\nundefined"

B "undefined" only

C "Hi" only

D No output

💡 Hint

Check step 4 output in the execution_table.

Concept Snapshot

Deno REPL lets you quickly test code interactively.
Start it by running 'deno' in terminal.
Type JavaScript/TypeScript code and press Enter.
Statements show 'undefined', expressions show results.
Exit with Ctrl+C twice.
Great for fast experiments and learning.

Full Transcript

The Deno REPL is a tool you start by typing 'deno' in your terminal. Once started, you can type JavaScript or TypeScript code and press Enter. If you define a variable like 'let x = 5;', the REPL shows 'undefined' because declarations do not return a value. When you type an expression like 'x * 2', the REPL evaluates it and shows the result, here '10'. You can also run commands like 'console.log("Hi")' which prints 'Hi' and then shows 'undefined'. To exit the REPL, press Ctrl+C twice. This interactive environment helps you quickly test and learn code snippets.

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