Why Hash Map Exists and What Problem It Solves in DSA Javascript - See It Work

Why Hash Map Exists and What Problem It Solves

📖 Scenario: Imagine you run a small library. You want to keep track of which book belongs to which shelf. You have many books, and you want to find any book quickly without searching every shelf.

🎯 Goal: You will create a simple way to store book names and their shelf numbers. Then, you will look up a book's shelf quickly using a hash map concept.

📋 What You'll Learn

Create a JavaScript object called library with exact book-shelf pairs

Create a variable called bookToFind with the exact book name to search

Use the library object to find the shelf number for bookToFind

Print the shelf number for the book

💡 Why This Matters

🌍 Real World

Hash maps are used in many apps to find information quickly, like looking up phone numbers, usernames, or product prices.

💼 Career

Understanding hash maps helps in software development jobs where fast data access is important, such as building search engines, databases, or caching systems.

1

DATA SETUP: Create the library object

Create a JavaScript object called library with these exact entries: 'The Hobbit': 5, '1984': 3, 'Hamlet': 7

DSA Javascript

// Create the library object with book names as keys and shelf numbers as values
// Your code here

Hint

Use curly braces {} to create an object. Put book names as keys and shelf numbers as values separated by colons.

2

CONFIGURATION: Choose the book to find

Create a variable called bookToFind and set it to the string '1984'

DSA Javascript

const library = {
  'The Hobbit': 5,
  '1984': 3,
  'Hamlet': 7
};

// Create the bookToFind variable with value '1984'
// Your code here

Hint

Use const bookToFind = '1984'; to store the book name you want to find.

3

CORE LOGIC: Find the shelf number using the hash map

Create a variable called shelfNumber and set it to the value in library for the key bookToFind

DSA Javascript

const library = {
  'The Hobbit': 5,
  '1984': 3,
  'Hamlet': 7
};

const bookToFind = '1984';

// Find the shelf number for bookToFind in library
// Your code here

Hint

Use bracket notation library[bookToFind] to get the shelf number for the book.

4

OUTPUT: Print the shelf number

Write console.log(shelfNumber); to print the shelf number of the book

DSA Javascript

const library = {
  'The Hobbit': 5,
  '1984': 3,
  'Hamlet': 7
};

const bookToFind = '1984';

const shelfNumber = library[bookToFind];

// Print the shelf number
// Your code here

Hint

Use console.log(shelfNumber); to show the shelf number in the console.

Practice

1. Why do we use a hash map in programming?
hashMap = {}

easy

A. To store key-value pairs for fast data lookup

B. To store data in a sorted order automatically

C. To perform mathematical calculations faster

D. To create a list of items without duplicates

Solution

1. Step 1: Understand the purpose of a hash map

   A hash map stores data as key-value pairs, allowing quick access to values using keys.

2. Step 2: Compare with other data structures

   Unlike lists or arrays, hash maps provide faster lookup by key, not by position or order.

3. Final Answer:

   To store key-value pairs for fast data lookup -> Option A

4. Quick Check:

   Hash map = fast key-value lookup [OK]

Hint: Hash maps store data by keys for quick access [OK]

Common Mistakes:

• Thinking hash maps sort data automatically
• Confusing hash maps with arrays or lists
• Assuming hash maps are for math calculations

2. Which of the following is the correct way to create an empty hash map (object) in JavaScript?

easy

A. const map = []

B. const map = {}

C. const map = () => {}

D. const map = new Map[]

Solution

1. Step 1: Recall JavaScript syntax for objects

   In JavaScript, curly braces {} create an empty object, which can be used as a hash map.

2. Step 2: Check other options

   Square brackets [] create arrays, () => {} is a function, and new Map[] is invalid syntax.

3. Final Answer:

   const map = {} -> Option B

4. Quick Check:

   Empty object = {} [OK]

Hint: Use curly braces {} to create empty hash maps in JavaScript [OK]

Common Mistakes:

• Using [] which creates an array, not a hash map
• Trying to use function syntax for hash map
• Incorrect new Map[] syntax

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

const map = {};
map['apple'] = 5;
map['banana'] = 3;
console.log(map['apple']);

medium

A. 5

B. Error

C. 3

D. undefined

Solution

1. Step 1: Assign values to keys in the hash map

   The code sets map['apple'] = 5 and map['banana'] = 3, storing these key-value pairs.

2. Step 2: Access the value for key 'apple'

   console.log(map['apple']) prints the value 5 stored at key 'apple'.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   map['apple'] = 5, output = 5 [OK]

Hint: Access value by key to get stored number [OK]

Common Mistakes:

• Expecting output to be 'undefined' if key exists
• Confusing values of different keys
• Thinking it causes an error

4. Find the error in this code that tries to use a hash map:

const map = {};
map.apple = 10;
console.log(map['applee']);

medium

A. Syntax error in object creation

B. Using dot notation instead of brackets

C. Cannot assign values to an empty object

D. Accessing a key that does not exist

Solution

1. Step 1: Check key assignment

   map.apple = 10 correctly assigns value 10 to key 'apple'.

2. Step 2: Check key access

   console.log(map['applee']) tries to access key 'applee' which was never assigned, so it returns undefined.

3. Final Answer:

   Accessing a key that does not exist -> Option D

4. Quick Check:

   Wrong key access = undefined [OK]

Hint: Check spelling of keys when accessing hash map values [OK]

Common Mistakes:

• Thinking dot notation is wrong here
• Expecting error on accessing missing key
• Assuming empty object cannot store values

5. You want to count how many times each word appears in an array using a hash map. Which approach correctly solves this problem?

const words = ['cat', 'dog', 'cat', 'bird'];
const counts = {};
// Fill counts here
console.log(counts);

hard

A. for (const w of words) { counts[w] = 1; }

B. for (const w of words) { counts[w] = counts[w] + 1; }

C. for (const w of words) { counts[w] = (counts[w] || 0) + 1; }

D. for (const w of words) { counts[w] += 1; }

Solution

1. Step 1: Understand counting logic

   We need to add 1 to the count for each word. If the word is new, start from 0.

2. Step 2: Analyze each option

   for (const w of words) { counts[w] = 1; } sets count to 1 always, ignoring repeats. for (const w of words) { counts[w] = counts[w] + 1; } tries to add 1 but counts[w] is undefined initially causing NaN. for (const w of words) { counts[w] = (counts[w] || 0) + 1; } uses (counts[w] || 0) + 1 to handle undefined keys correctly. for (const w of words) { counts[w] += 1; } tries to add 1 without initialization, causing NaN.

3. Final Answer:

   for (const w of words) { counts[w] = (counts[w] || 0) + 1; } -> Option C

4. Quick Check:

   Initialize with 0 or use || 0 before adding [OK]

Hint: Use (counts[key] || 0) + 1 to count safely [OK]

Common Mistakes:

• Not initializing counts before adding
• Overwriting counts instead of incrementing
• Assuming counts[w] is always defined