Writing first JavaScript program - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JavaScript code?

Look at this code snippet. What will it print to the console?

Javascript

console.log('Hello, ' + 'world!');

A Hello, world!

B Hello,world!

C Hello world!

D SyntaxError

💡 Hint

Remember how string concatenation works with the + operator.

The + operator joins the two strings exactly as written, including the comma and space inside the first string.

❓ Predict Output

intermediate

What will this code output?

Check this code and choose the correct console output.

Javascript

let x = 5;
console.log(x * 2);

A 10

B 52

C x * 2

D undefined

💡 Hint

Multiplying a number by 2 doubles it.

Variable x holds 5, so x * 2 equals 10, which is printed.

❓ Predict Output

advanced

What is the output of this code with a function?

What will this program print when run?

Javascript

function greet(name) {
  return `Hello, ${name}!`;
}
console.log(greet('Alice'));

A greet(Alice)

B Hello, name!

C Hello, Alice!

D SyntaxError

💡 Hint

The function uses template strings to insert the name.

The function greet returns a greeting with the name inserted. Calling greet('Alice') returns 'Hello, Alice!'.

❓ Predict Output

advanced

What does this code print?

Look at this code using let and const. What is the output?

Javascript

const greeting = 'Hi';
let name = 'Bob';
console.log(greeting + ', ' + name + '!');

A ReferenceError

B Hi, Bob!

C greeting, name!

D Hi Bob!

💡 Hint

Variables greeting and name hold strings that are joined with commas and spaces.

The + operator joins the strings exactly as written, including the comma and space.

❓ Predict Output

expert

What is the output of this code with asynchronous behavior?

What will this code print to the console?

Javascript

console.log('Start');
setTimeout(() => console.log('Middle'), 0);
console.log('End');

A

Middle
Start
End

B

Start
Middle
End

C

End
Start
Middle

D

Start
End
Middle

💡 Hint

setTimeout with 0 delay still runs after the current code finishes.

The synchronous console.log calls run first, printing 'Start' then 'End'. The setTimeout callback runs last, printing 'Middle'.

Practice

1. What does the console.log() function do in JavaScript?

easy

A. It changes the background color of the page.

B. It creates a pop-up alert on the screen.

C. It stops the program from running.

D. It shows messages in the browser console.

Solution

1. Step 1: Understand the purpose of console.log()

   This function is used to display messages or values in the browser's console, which helps developers see output or debug.

2. Step 2: Compare with other options

   Options A, B, and D describe different actions that console.log() does not perform.

3. Final Answer:

   It shows messages in the browser console. -> Option D

4. Quick Check:

   console.log() shows messages [OK]

Hint: Remember: console.log prints messages to the console [OK]

Common Mistakes:

• Thinking console.log creates alerts
• Confusing console.log with styling commands
• Assuming console.log stops the program

2. Which of the following is the correct syntax to print "Hello, World!" in JavaScript?

easy

A. print("Hello, World!");

B. console.log("Hello, World!");

C. echo "Hello, World!";

D. System.out.println("Hello, World!");

Solution

1. Step 1: Identify JavaScript syntax for output

   JavaScript uses console.log() to print messages to the console.

2. Step 2: Check other options for language mismatch

   A is Python style, B is Java style, C is PHP style, so they are incorrect for JavaScript.

3. Final Answer:

   console.log("Hello, World!"); -> Option B

4. Quick Check:

   JavaScript output uses console.log() [OK]

Hint: Use console.log() to print messages in JavaScript [OK]

Common Mistakes:

• Using print() instead of console.log()
• Confusing JavaScript with other languages
• Missing parentheses or semicolon

3. What will be the output of the following code?

console.log('Hi');
console.log('There');

medium

A. Hi
There

B. Hi There

C. Hi\nThere

D. HiThere

Solution

1. Step 1: Understand console.log behavior

   Each console.log() prints its message on a new line in the console.

2. Step 2: Analyze the output lines

   The first prints "Hi", then the second prints "There" on the next line, so output is two lines: "Hi" then "There".

3. Final Answer:

   Hi There -> Option A

4. Quick Check:

   console.log prints each message on a new line [OK]

Hint: Each console.log prints on a new line automatically [OK]

Common Mistakes:

• Thinking console.log joins messages on one line
• Confusing \n as literal text instead of newline
• Ignoring line breaks between calls

4. Find the error in this JavaScript code:

console.log('Hello World')

medium

A. console.log is misspelled

B. Incorrect quotes used

C. No error, code is correct

D. Missing semicolon at the end

Solution

1. Step 1: Check syntax correctness

   The code uses correct function name, quotes, and parentheses.

2. Step 2: Consider JavaScript semicolon rules

   Semicolons are optional at the end of statements in JavaScript, so missing semicolon is not an error.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   JavaScript allows optional semicolons [OK]

Hint: Semicolons are optional but recommended; no error here [OK]

Common Mistakes:

• Thinking missing semicolon causes error
• Confusing quote types as error
• Assuming console.log is misspelled

5. You want to write a JavaScript program that prints your name and age on separate lines. Which code will do this correctly?

hard

A. console.log('Name: John'); console.log('Age: 25');

B. console.log('Name: John\nAge: 25');

C. console.log('Name: John' + 'Age: 25');

D. console.log('Name: John Age: 25');

Solution

1. Step 1: Understand printing on separate lines

   Using two console.log() calls prints each message on its own line.

2. Step 2: Check other options for line breaks

   console.log('Name: John\nAge: 25'); uses \n which creates a newline in the output; console.log('Name: John' + 'Age: 25'); joins to 'Name: JohnAge: 25' on one line; console.log('Name: John Age: 25'); prints all on one line.

3. Final Answer:

   console.log('Name: John'); console.log('Age: 25'); -> Option A

4. Quick Check:

   Separate console.log calls print on separate lines [OK]

Hint: Use separate console.log() for each line [OK]

Common Mistakes:

• Using + without spaces causing joined text
• Thinking \n creates a newline (prints literal \n)
• Printing all info in one console.log without breaks