String Basics and Memory Representation in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this string concatenation code?

Consider the following JavaScript code that concatenates strings. What will be printed?

DSA Javascript

let a = 'Hello';
let b = 'World';
let c = a + ' ' + b;
console.log(c);

A "HelloWorld"

B "Hello World"

C "Hello World"

D "Hello+World"

💡 Hint

Look at how spaces are added between strings in concatenation.

The code concatenates 'Hello', a space ' ', and 'World'. So the output is 'Hello World'.

❓ Predict Output

intermediate

What is the output of this string indexing code?

What will this code print when accessing characters by index?

DSA Javascript

const str = 'Data';
console.log(str[0]);
console.log(str[3]);

A "D" and "a"

B "D" and "t"

C "a" and "a"

D "D" and "A"

💡 Hint

Remember string indexing starts at 0.

str[0] is 'D' (first letter), str[3] is 'a' (fourth letter).

🧠 Conceptual

advanced

How are strings stored in memory in JavaScript?

Which statement best describes how JavaScript stores strings in memory?

A Strings are stored as arrays of characters with fixed size in memory.

B Strings are immutable sequences of UTF-16 code units stored in contiguous memory locations.

C Strings are mutable objects that can be changed in place in memory.

D Strings are stored as linked lists of characters with pointers to each character.

💡 Hint

Think about immutability and encoding in JavaScript strings.

JavaScript strings are immutable and stored as sequences of UTF-16 code units in contiguous memory.

❓ Predict Output

advanced

What is the output of this code using string methods?

What will this code print?

DSA Javascript

const s = 'Memory';
const result = s.slice(1, 4);
console.log(result);

A "Mem"

B "mory"

C "ome"

D "emo"

💡 Hint

slice(start, end) extracts characters from start up to but not including end.

slice(1,4) extracts characters at indices 1,2,3 which are 'e','m','o'.

❓ Predict Output

expert

What is the output of this code showing string immutability?

What will this code print?

DSA Javascript

let str = 'Hello';
str[0] = 'Y';
console.log(str);

A "Hello"

B "Yello" with an error

C "ello"

D "Yello"

💡 Hint

Strings cannot be changed by assigning to an index.

Strings are immutable, so str[0] = 'Y' does nothing. str remains 'Hello'.

Practice

1. What does the term immutable mean when talking about strings in JavaScript?

easy

A. Strings cannot be changed after they are created

B. Strings can be changed anytime

C. Strings are stored as numbers

D. Strings are always empty

Solution

1. Step 1: Understand string immutability

   In JavaScript, once a string is created, its content cannot be changed directly.

2. Step 2: Compare options with definition

   Strings cannot be changed after they are created - this matches the definition.

3. Final Answer:

   Strings cannot be changed after they are created -> Option A

4. Quick Check:

   Immutable means unchangeable = D [OK]

Hint: Remember: strings can't be changed once made [OK]

Common Mistakes:

• Thinking strings can be changed like arrays
• Confusing immutability with emptiness
• Assuming strings are stored as numbers

2. Which of the following is the correct way to create a string in JavaScript?

easy

A. const str = Hello World;

B. const str = (Hello World);

C. const str = `Hello World;

D. const str = 'Hello World';

Solution

1. Step 1: Check string syntax rules

   Strings must be enclosed in quotes: single (' '), double (" "), or backticks (` `).

2. Step 2: Validate each option

   const str = 'Hello World'; uses single quotes correctly. const str = Hello World; misses quotes. const str = `Hello World; misses closing backtick. const str = (Hello World); uses parentheses incorrectly.

3. Final Answer:

   const str = 'Hello World'; -> Option D

4. Quick Check:

   Quotes needed for strings = B [OK]

Hint: Strings need quotes around text [OK]

Common Mistakes:

• Forgetting quotes around string text
• Using mismatched or missing quotes
• Using parentheses instead of quotes

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

let s = 'hello';
s[0] = 'H';
console.log(s);

medium

A. hello

B. Hello

C. undefined

D. Error

Solution

1. Step 1: Understand string immutability in code

   Strings cannot be changed by assigning to an index like s[0] = 'H'. This operation is ignored.

2. Step 2: Check what console.log prints

   The original string 'hello' remains unchanged and is printed.

3. Final Answer:

   hello -> Option A

4. Quick Check:

   Strings unchanged by index assignment = A [OK]

Hint: Strings ignore index assignment, original stays same [OK]

Common Mistakes:

• Expecting first letter to change
• Thinking strings behave like arrays
• Assuming error will be thrown

4. Find the error in this code snippet that tries to change a string character:

let word = 'code';
word[2] = 'd';
console.log(word);

medium

A. Variable word is not declared

B. Strings are immutable; cannot assign to index

C. Missing semicolon after declaration

D. console.log syntax is wrong

Solution

1. Step 1: Analyze string modification attempt

   Assigning to a string index like word[2] = 'd' does not change the string because strings are immutable.

2. Step 2: Check other options for errors

   Semicolons are optional in JavaScript, variable is declared, and console.log syntax is correct.

3. Final Answer:

   Strings are immutable; cannot assign to index -> Option B

4. Quick Check:

   Immutable strings block index assignment = C [OK]

Hint: Strings can't be changed by index assignment [OK]

Common Mistakes:

• Thinking semicolon is mandatory here
• Assuming variable is undeclared
• Believing console.log syntax is incorrect

5. Given the code below, what will be the output?

const str1 = 'abc';
const str2 = str1 + 'd';
console.log(str1);
console.log(str2);

Choose the correct output representation.

hard

A. Both lines print abcd

B. abcd and abc printed on separate lines

C. abc and abcd printed on separate lines

D. Error because strings are immutable

Solution

1. Step 1: Understand string concatenation and immutability

   str1 remains 'abc' because strings are immutable. str2 is a new string 'abcd' created by adding 'd' to str1.

2. Step 2: Check console.log outputs

   console.log(str1) prints abc. console.log(str2) prints abcd.

3. Final Answer:

   abc and abcd printed on separate lines -> Option C

4. Quick Check:

   Original unchanged, new string created = A [OK]

Hint: Concatenation creates new string, original stays same [OK]

Common Mistakes:

• Thinking original string changes after concatenation
• Expecting error due to immutability
• Confusing order of printed strings