String Basics and Memory Representation in DSA Javascript - Time & Space Complexity

Time Complexity: String Basics and Memory Representation

O(n^2)

Understanding Time Complexity

We want to understand how the time to work with strings changes as the string gets longer.

How does the length of a string affect the time to do basic operations?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const str = "hello world";
let result = "";
for (let i = 0; i < str.length; i++) {
  result += str[i];
}
console.log(result);
    

This code copies each character from one string to another by looping through the original string.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each character of the string.
• How many times: Once for each character in the string (n times).

How Execution Grows With Input

As the string gets longer, the loop runs more times, so the work grows with the string length.

Input Size (n)	Approx. Operations
10	About 10 character copies
100	About 100 character copies
1000	About 1000 character copies

Pattern observation: The number of operations grows directly with the string length.

Final Time Complexity

Time Complexity: O(n^2)

This means the time to copy the string grows quadratically with the string length due to repeated string concatenation.

Common Mistake

[X] Wrong: "Copying a string is instant and does not depend on its length."

[OK] Correct: Each character must be handled one by one, so longer strings take more time.

Interview Connect

Understanding how string operations scale helps you write efficient code and explain your choices clearly in interviews.

Self-Check

"What if we used string concatenation inside the loop versus using an array and joining at the end? How would the time complexity change?"

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