How Strings Work Differently Across Languages in DSA Javascript - Debugging & Practice

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JavaScript string operation?

Consider the following JavaScript code snippet. What will be printed to the console?

DSA Javascript

const str = 'hello';
const result = str[0] + str.length + str[str.length - 1];
console.log(result);

A "5ho"

B "h5o"

C "hello"

D "hlo"

💡 Hint

Remember that str.length is a number and adding it to strings concatenates as string.

str[0] is 'h', str.length is 5, and str[str.length - 1] is 'o'. Adding them as strings results in 'h5o'.

❓ Predict Output

intermediate

What does this code print regarding string immutability?

Look at this JavaScript code. What is the output after running it?

DSA Javascript

let s = 'abc';
s[0] = 'z';
console.log(s);

A "zbc"

B "z"

C "abc"

D TypeError

💡 Hint

Strings in JavaScript cannot be changed by assigning to an index.

Strings are immutable in JavaScript, so s[0] = 'z' does nothing. The original string remains 'abc'.

❓ Predict Output

advanced

What is the output of this string comparison in JavaScript?

What will this code print to the console?

DSA Javascript

console.log('a' < 'b');
console.log('a' < 'A');
console.log('abc' < 'abd');

A

true
false
true

B

false
true
false

C

true
true
false

D

false
false
true

💡 Hint

JavaScript compares strings lexicographically using Unicode values.

'a' < 'b' is true because 'a' comes before 'b'. 'a' < 'A' is false because lowercase 'a' has higher Unicode than uppercase 'A'. 'abc' < 'abd' is true because 'c' < 'd'.

❓ Predict Output

advanced

What is the output of this code involving Unicode characters?

What will this JavaScript code print?

DSA Javascript

const s = '𝟘𝟙𝟚';
console.log(s.length);
console.log(s[0]);

A

3
�

B

3
𝟘

C

6
𝟘

D

6
�

💡 Hint

Some Unicode characters use two code units in JavaScript strings.

Each character '𝟘', '𝟙', '𝟚' is represented by two UTF-16 code units, so length is 6. s[0] returns the first code unit, which is a half of the character, shown as \uD835.

🧠 Conceptual

expert

Why do some languages treat strings as arrays of characters while others do not?

Which of the following best explains why JavaScript strings behave differently from languages like C when accessing characters by index?

A JavaScript strings are immutable and use UTF-16 encoding, so indexing returns code units, not characters.

B JavaScript strings are mutable arrays of bytes, so indexing modifies the string directly.

C JavaScript strings are stored as linked lists of characters, making indexing slow.

D JavaScript strings are stored as ASCII arrays, so indexing always returns ASCII codes.

💡 Hint

Think about string encoding and mutability in JavaScript.

JavaScript strings are immutable sequences of UTF-16 code units. Indexing returns code units, which may be half of a Unicode character. This differs from languages like C where strings are mutable arrays of bytes.

Practice

1. In JavaScript, which of the following statements about strings is true?

easy

A. Strings are immutable, meaning you cannot change a character directly.

B. Strings are mutable, so you can change characters by index.

C. Strings are stored as arrays of characters that can be modified.

D. Strings are objects that allow direct character assignment.

Solution

1. Step 1: Understand string mutability in JavaScript

   JavaScript strings cannot be changed after creation; they are immutable.

2. Step 2: Compare with other options

   Options A, B, and C suggest mutability or direct modification, which is incorrect.

3. Final Answer:

   Strings are immutable, meaning you cannot change a character directly. -> Option A

4. Quick Check:

   JavaScript strings are immutable = D [OK]

Hint: Remember: JavaScript strings cannot be changed character by character [OK]

Common Mistakes:

• Thinking strings behave like arrays and can be changed by index
• Confusing string immutability with array mutability
• Assuming strings are objects with mutable properties

2. Which of the following is the correct way to get the length of a string str in JavaScript?

easy

A. str.size

B. str.length()

C. length(str)

D. str.length

Solution

1. Step 1: Recall JavaScript string length syntax

   In JavaScript, string length is accessed as a property without parentheses: str.length

2. Step 2: Check other options for syntax errors

   str.length() uses parentheses which is incorrect; C and D are not valid JavaScript syntax for string length.

3. Final Answer:

   str.length -> Option D

4. Quick Check:

   Use property .length without () = A [OK]

Hint: Use .length property without parentheses for string length [OK]

Common Mistakes:

• Using parentheses like a function call (str.length())
• Confusing with other languages that use length() function
• Trying to use str.size which is invalid in JavaScript

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

const s = 'hello';
console.log(s[1]);
s[1] = 'a';
console.log(s[1]);

medium

A. e\ne

B. e\na

C. undefined\na

D. undefined\ne

Solution

1. Step 1: Check first console.log(s[1])

   Index 1 of 'hello' is 'e', so first output is 'e'.

2. Step 2: Attempt to change s[1] and check second console.log(s[1])

   Strings are immutable, so s[1] = 'a' does nothing; second output remains 'e'.

3. Final Answer:

   e\ne -> Option A

4. Quick Check:

   Immutable strings keep original chars = B [OK]

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

Common Mistakes:

• Expecting s[1] to change to 'a'
• Confusing string mutability with arrays
• Assuming assignment to string index works

4. Identify the error in the following JavaScript code snippet:

let str = 'world';
str[0] = 'W';
console.log(str);

medium

A. No error; output is 'World'

B. No error; output is 'world'

C. TypeError because strings are immutable

D. SyntaxError due to invalid assignment

Solution

1. Step 1: Understand string immutability and assignment

   Assigning to str[0] does not change the string; no error is thrown.

2. Step 2: Check console.log output

   Since assignment fails silently, output remains original string 'world'.

3. Final Answer:

   No error; output is 'world' -> Option B

4. Quick Check:

   Immutable strings ignore index assignment = C [OK]

Hint: Index assignment on strings fails silently, no error thrown [OK]

Common Mistakes:

• Expecting a TypeError on index assignment
• Thinking string changes after index assignment
• Confusing with languages that allow mutable strings

5. You want to replace the first character of a string str with an uppercase letter in JavaScript. Which approach correctly achieves this?

hard

A. str.replace(0, str[0].toUpperCase());

B. str[0] = str[0].toUpperCase();

C. str = str[0].toUpperCase() + str.slice(1);

D. str.toUpperCase()[0] + str.slice(1);

Solution

1. Step 1: Understand string immutability and modification

   Direct assignment to str[0] is invalid; strings are immutable.

2. Step 2: Check each option for correct string replacement

   str = str[0].toUpperCase() + str.slice(1); creates a new string by uppercasing first char and adding rest; this works correctly.

3. Step 3: Analyze other options

   B tries direct assignment (invalid), C uses replace incorrectly (replace needs substring, not index), D uppercases whole string then concatenates first char with rest but doesn't reassign to str (str unchanged).

4. Final Answer:

   str = str[0].toUpperCase() + str.slice(1); -> Option C

5. Quick Check:

   Use new string with uppercase first char + rest = A [OK]

Hint: Build new string; can't change characters directly [OK]

Common Mistakes:

• Trying to assign directly to str[0]
• Using replace with index instead of substring
• Uppercasing whole string instead of first char only