Generic class syntax in Typescript - Time & Space Complexity

Time Complexity: Generic class syntax

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a generic class changes as we use it with different amounts of data.

How does the size of the input affect the work done inside the generic class?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Box<T> {
  items: T[] = [];

  add(item: T) {
    this.items.push(item);
  }

  get(index: number): T | undefined {
    return this.items[index];
  }
}
    

This code defines a generic class that stores items and allows adding and retrieving them by index.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Adding an item to the array using push.
• How many times: Once per add call, no loops inside add or get methods.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	10 push operations
100	100 push operations
1000	1000 push operations

Pattern observation: Each add call does a simple push, so operations grow directly with the number of items added.

Final Time Complexity

Time Complexity: O(n)

This means the time to add n items grows in a straight line with n; doubling items roughly doubles the time.

Common Mistake

[X] Wrong: "Using a generic class makes operations slower because of extra type handling."

[OK] Correct: Generics in TypeScript are erased at runtime, so they do not add extra time cost to operations.

Interview Connect

Understanding how generic classes behave helps you explain how your code scales and stays efficient when handling different data types.

Self-Check

"What if the add method searched for duplicates before adding? How would the time complexity change?"

Practice

1. What does the <T> mean in a TypeScript generic class declaration like class Box<T> {}?

easy

A. It declares a placeholder type that can be replaced with any type when creating an instance.

B. It defines a fixed type named T that cannot be changed.

C. It is a syntax error and should be removed.

D. It specifies that the class only works with numbers.

Solution

1. Step 1: Understand generic type parameter

   The <T> after the class name declares a generic type parameter named T, which acts as a placeholder for any type.

2. Step 2: Purpose of generic classes

   This allows the class to work flexibly with any type specified when creating an instance, making the code reusable and type-safe.

3. Final Answer:

   It declares a placeholder type that can be replaced with any type when creating an instance. -> Option A

4. Quick Check:

   Generic <T> = placeholder type [OK]

Hint: Generic <T> means flexible type placeholder [OK]

Common Mistakes:

• Thinking <T> fixes the type permanently
• Confusing generic syntax with inheritance
• Assuming <T> is a variable or value

2. Which of the following is the correct syntax to declare a generic class named Container with a type parameter T?

easy

A. class Container[T] {}

B. class Container(T) {}

C. class Container {T} {}

D. class Container<T> {}

Solution

1. Step 1: Recall generic class syntax

   In TypeScript, generic classes use angle brackets <> to declare type parameters after the class name.

2. Step 2: Match correct syntax

   The correct syntax is class Container<T> {}. Other options use parentheses, curly braces, or square brackets incorrectly.

3. Final Answer:

   class Container<T> {} -> Option D

4. Quick Check:

   Generic class uses <T> syntax [OK]

Hint: Generic classes use angle brackets <T> after name [OK]

Common Mistakes:

• Using parentheses instead of angle brackets
• Placing type parameter inside class body
• Using square brackets which are for arrays

3. What is the output of this TypeScript code?

class Box<T> {
  content: T;
  constructor(value: T) {
    this.content = value;
  }
  getContent(): T {
    return this.content;
  }
}

const box = new Box<string>('hello');
console.log(box.getContent());

medium

A. Box { content: 'hello' }

B. undefined

C. hello

D. TypeError at runtime

Solution

1. Step 1: Understand class instantiation

   The class Box<T> is instantiated with T = string, so content is a string holding 'hello'.

2. Step 2: Trace method call

   The method getContent() returns the stored content, which is 'hello'. The console.log prints this string.

3. Final Answer:

   hello -> Option C

4. Quick Check:

   Generic class returns stored value 'hello' [OK]

Hint: Generic class returns stored value of specified type [OK]

Common Mistakes:

• Expecting object print instead of string
• Confusing property with method output
• Thinking generic causes runtime error

4. Identify the error in this generic class code:

class Pair<T, U> {
  first: T;
  second: U;
  constructor(first: T, second: T) {
    this.first = first;
    this.second = second;
  }
}

medium

A. Constructor parameters should be (first: T, second: U) to match property types.

B. Generic parameters should be declared as <T> only, not .

C. Properties first and second must have the same type T.

D. Constructor cannot have parameters in a generic class.

Solution

1. Step 1: Compare constructor parameters with property types

   The class declares two generic types T and U, with properties first: T and second: U.

2. Step 2: Identify mismatch in constructor

   The constructor incorrectly uses second: T instead of second: U, causing a type mismatch.

3. Final Answer:

   Constructor parameters should be (first: T, second: U) to match property types. -> Option A

4. Quick Check:

   Constructor params must match generic property types [OK]

Hint: Match constructor param types to generic properties [OK]

Common Mistakes:

• Using same type for both parameters ignoring generics
• Removing one generic parameter incorrectly
• Thinking constructor can't have parameters in generics

5. You want to create a generic class Dictionary<K, V> that stores key-value pairs. Which implementation correctly uses generics to store and retrieve values by key?

class Dictionary<K, V> {
  private items: Map<K, V> = new Map();

  add(key: K, value: V): void {
    this.items.set(key, value);
  }

  get(key: K): V | undefined {
    return this.items.get(key);
  }
}

hard

A. Generic types cannot be used with Map in TypeScript.

B. This implementation correctly uses generics to store and retrieve values by key.

C. The add method should return the value added, not void.

D. The Map should be declared as Map<V, K> to match key-value order.

Solution

1. Step 1: Analyze Map usage with generics

   The class uses Map<K, V> to store keys of type K and values of type V, which is correct.

2. Step 2: Check methods for correct generic usage

   The add method sets a key-value pair, and get retrieves the value by key, both using the generic types properly.

3. Final Answer:

   This implementation correctly uses generics to store and retrieve values by key. -> Option B

4. Quick Check:

   Generic Map<K, V> with add/get methods = correct [OK]

Hint: Use Map with add/get methods for generic dictionary [OK]

Common Mistakes:

• Swapping key and value types in Map declaration
• Expecting add to return a value unnecessarily
• Believing generics can't be used with built-in types