Why Hash Map Exists and What Problem It Solves in DSA Typescript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple key-value store using a hash map.

DSA Typescript

const map = new Map<string, number>();
map.set('apple', [1]);
console.log(map.get('apple'));

Drag options to blanks, or click blank then click option'

A5

B'five'

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number as the value.

Using a boolean or null which are not expected here.

The value stored for the key 'apple' should be a number, here 5.

2fill in blank

medium

Complete the code to check if a key exists in the hash map.

DSA Typescript

if (map.[1]('apple')) {
  console.log('Key exists');
}

Drag options to blanks, or click blank then click option'

Acontains

Bexists

Chas

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'find' which is not a Map method.

Using 'contains' or 'exists' which do not exist on Map.

The correct method to check if a key exists in a Map is has.

3fill in blank

hard

Fix the error in the code to retrieve a value safely from the hash map.

DSA Typescript

const value = map.get('banana');
if (value [1] undefined) {
  console.log('Value:', value);
} else {
  console.log('No value found');
}

Drag options to blanks, or click blank then click option'

A!==

B!=

C==

D===

Attempts:

3 left

💡 Hint

Common Mistakes

Using == or != which allow type coercion.

Using === which checks equality, not inequality.

Use strict inequality !== to check that value is not undefined.

4fill in blank

hard

Fill both blanks to create a hash map from an array of keys with initial value 0.

DSA Typescript

const keys = ['a', 'b', 'c'];
const map = new Map<string, number>(keys.map(key => [key, [1]]));
console.log(map.get([2]));

Drag options to blanks, or click blank then click option'

A0

B'a'

C'b'

D'c'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of number 0 as initial value.

Using a key not in the keys array.

Each key maps to 0 initially, and we check the value for key 'a'.

5fill in blank

hard

Fill all three blanks to update a hash map value by incrementing it safely.

DSA Typescript

const count = map.get('x') [1] 0;
map.set('x', count [2] 1);
console.log(map.get('x') === [3]);

Drag options to blanks, or click blank then click option'

A||

B+

C1

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Not handling undefined value before increment.

Using subtraction instead of addition.

Checking against wrong expected value.

Use || to default to 0 if undefined, then add 1, and check if result is 1.

Practice

1. Why do we use a Hash Map instead of a simple list to store key-value pairs?

easy

A. Because it allows fast lookup of values by keys

B. Because it stores data in sorted order automatically

C. Because it uses less memory than a list

D. Because it can only store numbers

Solution

1. Step 1: Understand the problem with lists

   Lists require checking each item one by one to find a value by key, which is slow for large data.

2. Step 2: How hash maps solve this

   Hash maps use a special function to find the exact place of a key quickly, making lookup fast.

3. Final Answer:

   Because it allows fast lookup of values by keys -> Option A

4. Quick Check:

   Fast key lookup = Because it allows fast lookup of values by keys [OK]

Hint: Hash maps speed up key-based searches [OK]

Common Mistakes:

• Thinking hash maps sort data automatically
• Believing hash maps use less memory always
• Assuming hash maps only store numbers

2. Which of the following is the correct way to create a hash map (Map) in TypeScript?

easy

A. const map = new Map();

B. const map = Map();

C. const map = new map();

D. const map = new Map(); string, number;

Solution

1. Step 1: Recall TypeScript Map syntax

   In TypeScript, a Map is created using new Map<KeyType, ValueType>().

2. Step 2: Check each option

   const map = new Map(); uses correct syntax with new Map<string, number>(). Others have syntax errors or wrong casing.

3. Final Answer:

   const map = new Map<string, number>(); -> Option A

4. Quick Check:

   Correct Map creation syntax = const map = new Map<string, number>(); [OK]

Hint: Use 'new Map<KeyType, ValueType>()' to create hash maps [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using lowercase 'map' instead of 'Map'
• Incorrect generic syntax

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

const map = new Map();
map.set('apple', 5);
map.set('banana', 3);
map.set('apple', 10);
console.log(map.get('apple'));

medium

A. Error

B. 5

C. undefined

D. 10

Solution

1. Step 1: Understand map.set behavior

   When setting a key that already exists, the value is updated to the new one.

2. Step 2: Trace the code

   First 'apple' is set to 5, then updated to 10. So map.get('apple') returns 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Updated value for 'apple' = 10 [OK]

Hint: Last set value for a key overwrites previous [OK]

Common Mistakes:

• Assuming first value remains
• Thinking get returns undefined if key repeated
• Expecting error on duplicate keys

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

const map = new Map();
map['key'] = 'value';
console.log(map.get('key'));

medium

A. map.get() cannot be used to retrieve values

B. Map constructor requires key and value types

C. Using bracket notation to add key-value pairs does not work with Map

D. Map keys must be numbers only

Solution

1. Step 1: Understand how to add entries to Map

   Map entries must be added using map.set(key, value), not bracket notation.

2. Step 2: Analyze the code behavior

   Using map['key'] = 'value' adds a property to the object, not a Map entry, so map.get('key') returns undefined.

3. Final Answer:

   Using bracket notation to add key-value pairs does not work with Map -> Option C

4. Quick Check:

   Use map.set() to add entries [OK]

Hint: Use map.set() to add entries, not bracket notation [OK]

Common Mistakes:

• Adding entries with bracket notation
• Expecting map.get() to find bracket-added keys
• Thinking Map keys must be numbers

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

hard

A. Use a Set to store words and count size for each word

B. Use a Map, for each word check if it exists, then increment or set to 1

C. Use a Map with word as value and count as key

D. Use an array and push words, then count duplicates manually later

Solution

1. Step 1: Understand the counting problem

   We need to store each word as a key and count as a value, so Map<string, number> fits well.

2. Step 2: How to update counts

   For each word, check if it exists in the map; if yes, increment count, else set count to 1.

3. Step 3: Why other options fail

   Use an array and push words, then count duplicates manually later is inefficient and manual. Use a Map with word as value and count as key swaps key and value wrongly. Use a Set to store words and count size for each word's Set only stores unique words without counts.

4. Final Answer:

   Use a Map<string, number>, for each word check if it exists, then increment or set to 1 -> Option B

5. Quick Check:

   Counting words = Map with word keys and number counts [OK]

Hint: Map keys = words, values = counts; increment or set 1 [OK]

Common Mistakes:

• Using Set which can't count duplicates
• Swapping key and value types in Map
• Counting manually with arrays inefficiently