Basic input concepts (prompt overview) in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this prompt input code?

Consider this JavaScript code that asks the user for their name and then greets them. What will be shown in the console if the user enters Alex?

Javascript

const name = prompt('What is your name?');
console.log(`Hello, ${name}!`);

A Hello, name!

B Hello, Alex!

C What is your name?

D undefined

💡 Hint

The prompt function returns the text the user types.

The prompt function shows a box asking for input. The input is stored in the variable 'name'. Then console.log prints 'Hello, ' plus the input plus '!'. So if the user types 'Alex', the output is 'Hello, Alex!'.

❓ Predict Output

intermediate

What happens if the user presses Cancel in prompt?

Look at this code snippet. What will be the value of answer if the user presses Cancel in the prompt box?

Javascript

const answer = prompt('Type something or press Cancel');
console.log(answer);

A Throws an error

B "" (empty string)

C undefined

D null

💡 Hint

Cancel returns a special value different from empty string.

When the user presses Cancel, prompt returns null, which is logged to the console.

🧠 Conceptual

advanced

Why should you convert prompt input to a number?

The prompt function always returns a string. Why is it important to convert this string to a number when expecting numeric input?

A Because strings cannot be stored in variables

B Because prompt returns undefined for numbers

C Because arithmetic operations on strings can produce unexpected results

D Because numbers are always returned as null

💡 Hint

Think about what happens if you add two strings that look like numbers.

If you don't convert the string to a number, adding two inputs will concatenate them as strings instead of adding numerically.

❓ Predict Output

advanced

What is the output of this code with numeric input?

What will be the output if the user enters 5 in the prompt?

Javascript

const input = prompt('Enter a number');
const result = input * 2;
console.log(result);

A 10

B "55"

C undefined

D NaN

💡 Hint

Multiplying a string that looks like a number by a number converts it automatically.

JavaScript converts the string '5' to number 5 when using the * operator, so 5 * 2 = 10.

❓ Predict Output

expert

What error does this code produce?

What error will this code cause when run in a browser?

Javascript

const name = prompt('Enter your name');
alert('Hello, ' + name.toUpperCase());

A TypeError: Cannot read property 'toUpperCase' of null

B SyntaxError: Unexpected token

C ReferenceError: name is not defined

D No error, alerts 'Hello, NAME'

💡 Hint

What happens if the user presses Cancel and name is null?

If the user presses Cancel, prompt returns null. Calling toUpperCase() on null causes a TypeError.