Method overriding with types in Typescript - Time & Space Complexity

Time Complexity: Method overriding with types

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run code changes when using method overriding with types in TypeScript.

Specifically, we ask: how does the program's work grow as input or calls increase?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Animal {
  speak(): string {
    return "...";
  }
}

class Dog extends Animal {
  speak(): string {
    return "Woof!";
  }
}

function makeSpeak(animal: Animal) {
  return animal.speak();
}

const dog = new Dog();
makeSpeak(dog);
    

This code shows a base class with a method, a subclass that overrides it, and a function calling the method on an object.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the speak method on an object.
• How many times: Once per call to makeSpeak. No loops or recursion here.

How Execution Grows With Input

Since there is no loop or repeated calls inside the snippet, the work grows directly with how many times makeSpeak is called.

Input Size (n)	Approx. Operations
10	10 method calls
100	100 method calls
1000	1000 method calls

Pattern observation: The time grows linearly with the number of calls.

Final Time Complexity

Time Complexity: O(n)

This means if you call the method n times, the total work grows roughly in direct proportion to n.

Common Mistake

[X] Wrong: "Overriding a method makes the program slower by adding hidden loops or extra work."

[OK] Correct: Overriding just changes which code runs when the method is called. It does not add loops or repeated work by itself.

Interview Connect

Understanding how method calls and overriding affect performance helps you explain code behavior clearly and confidently in interviews.

Self-Check

"What if the speak method called itself recursively? How would the time complexity change?"

Practice

1.

What does method overriding mean in TypeScript?

easy

A. A child class provides its own version of a method from the parent class.

B. A child class deletes a method from the parent class.

C. A parent class changes a method from the child class.

D. A method is called without any arguments.

Solution

1. Step 1: Understand method overriding

   Method overriding means a child class replaces a method defined in its parent class with its own version.

2. Step 2: Identify correct description

   A child class provides its own version of a method from the parent class. correctly describes this behavior as the child class providing its own method version.

3. Final Answer:

   A child class provides its own version of a method from the parent class. -> Option A

4. Quick Check:

   Method overriding = child method replaces parent method [OK]

Hint: Child class method replaces parent method behavior [OK]

Common Mistakes:

• Thinking parent changes child methods
• Confusing overriding with deleting methods
• Believing overriding means calling method without arguments

2.

Which of the following is the correct syntax for overriding a method greet() in a child class in TypeScript?

class Parent {
  greet(): string {
    return "Hello";
  }
}

class Child extends Parent {
  ?
}

easy

A. greet() { return 10; }

B. greet(): number { return 5; }

C. greet(name: string): string { return `Hi, ${name}`; }

D. greet(): string { return "Hi"; }

Solution

1. Step 1: Check method signature compatibility

   The child method must have the same name and compatible return type as the parent method. Parent returns string and takes no parameters.

2. Step 2: Identify correct override

   greet(): string { return "Hi"; } matches the parent's method signature exactly (no parameters, returns string). Others differ in return type or parameters.

3. Final Answer:

   greet(): string { return "Hi"; } -> Option D

4. Quick Check:

   Override method signature must match parent [OK]

Hint: Override method must match parent's name and return type [OK]

Common Mistakes:

• Changing return type to number
• Adding parameters not in parent method
• Omitting return type causing implicit any

3.

What will be the output of this TypeScript code?

class Animal {
  speak(): string {
    return "Animal sound";
  }
}

class Dog extends Animal {
  speak(): string {
    return "Bark";
  }
}

const pet: Animal = new Dog();
console.log(pet.speak());

medium

A. "Bark"

B. Error: Method not found

C. undefined

D. "Animal sound"

Solution

1. Step 1: Understand polymorphism with method overriding

   Even though the variable type is Animal, the actual object is Dog. The overridden method in Dog is called.

2. Step 2: Determine method output

   Dog's speak() returns "Bark", so console.log prints "Bark".

3. Final Answer:

   "Bark" -> Option A

4. Quick Check:

   Overridden method runs on actual object type [OK]

Hint: Actual object method runs, not variable type method [OK]

Common Mistakes:

• Thinking parent method runs due to variable type
• Expecting undefined because of type mismatch
• Assuming runtime error for overridden method

4.

Identify the error in this TypeScript code that tries to override a method:

class Vehicle {
  move(distance: number): void {
    console.log(`Moved ${distance} meters`);
  }
}

class Car extends Vehicle {
  move(distance: string): void {
    console.log(`Car moved ${distance}`);
  }
}

medium

A. Child class cannot override methods from parent class.

B. Return type void is not allowed in overriding.

C. Parameter type in child method must match parent method's parameter type.

D. Method name must be different in child class.

Solution

1. Step 1: Check method signature compatibility

   Parent method move() takes a number parameter. Child method move() takes a string parameter, which is incompatible.

2. Step 2: Identify correct error cause

   Parameter type in child method must match parent method's parameter type. correctly states this.

3. Final Answer:

   Parameter type in child method must match parent method's parameter type. -> Option C

4. Quick Check:

   Parameter types must be compatible in overriding [OK]

Hint: Parameter types must match exactly for overriding [OK]

Common Mistakes:

• Changing parameter types in child method
• Thinking return type void is invalid
• Believing method names must differ

5.

Given these classes, which override is valid and why?

class Base {
  calculate(value: number): number {
    return value * 2;
  }
}

class Derived extends Base {
  calculate(value: number): number | string {
    if (value > 10) return "Too large";
    return value * 3;
  }
}

hard

A. Valid because return type includes parent's return type (number) plus string.

B. Invalid because return type must exactly match parent's return type.

C. Invalid because method name changed.

D. Valid because child method can return any type.

Solution

1. Step 1: Check return type compatibility in overriding

   Parent method returns number only. Child method returns number or string, which is not compatible because string is not assignable to number.

2. Step 2: Understand TypeScript's strict method overriding rules

   TypeScript requires the child method's return type to be assignable to the parent's return type, not broader. Invalid because return type must exactly match parent's return type. correctly states this.

3. Final Answer:

   Invalid because return type must exactly match parent's return type. -> Option B

4. Quick Check:

   Return type must be compatible, not broader [OK]

Hint: Child return type must be assignable to parent's return type [OK]

Common Mistakes:

• Allowing broader return types in child method
• Ignoring method name consistency
• Assuming any return type is allowed in override