String Basics and Memory Representation in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a string in programming?

A string is a sequence of characters used to store text. It can include letters, numbers, and symbols.

intermediate

How are strings stored in memory?

Strings are stored as a sequence of characters in contiguous memory locations, each character usually taking one or more bytes depending on encoding.

intermediate

What does it mean that strings are immutable in JavaScript?

It means once a string is created, it cannot be changed. Any operation that modifies a string creates a new string instead.

intermediate

Explain the difference between string length and memory size.

String length is the number of characters in the string. Memory size is how many bytes those characters take in memory, which can be more due to encoding.

advanced

What is UTF-16 encoding in JavaScript strings?

UTF-16 is the encoding JavaScript uses to store strings, where most characters use 2 bytes, but some special characters use 4 bytes (surrogate pairs).

Which of the following best describes a string?

A A collection of boolean values

B A single number

C A sequence of characters

D A type of function

A string is a sequence of characters used to represent text.

In JavaScript, what happens when you change a string?

A The string becomes a number

B The original string is modified in place

C The string is deleted

D A new string is created with the change

Strings are immutable in JavaScript, so changes create a new string.

What does string length represent?

A Number of characters in the string

B Number of lines in the string

C Number of words in the string

D Number of bytes used in memory

String length counts how many characters are in the string.

Which encoding does JavaScript use for strings?

A ASCII

B UTF-16

C UTF-8

D ISO-8859-1

JavaScript uses UTF-16 encoding for strings.

How are characters stored in memory for strings?

A In contiguous memory locations

B Randomly scattered

C In a linked list

D In a hash table

Characters in strings are stored in contiguous memory locations.

Describe how strings are stored and represented in memory.

Think about how each character is saved one after another and how encoding affects size.

Explain what it means that strings are immutable in JavaScript and why it matters.

Consider what happens when you try to change a string's content.

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