Why typed classes matter in Typescript - Performance Analysis

Time Complexity: Why typed classes matter

O(n)

Understanding Time Complexity

When using typed classes in TypeScript, it is important to understand how the program's work grows as the input changes.

We want to see how the time to create and use typed classes changes when we have more data or objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class User {
  constructor(public id: number, public name: string) {}
}

function createUsers(n: number): User[] {
  const users: User[] = [];
  for (let i = 0; i < n; i++) {
    users.push(new User(i, `User${i}`));
  }
  return users;
}
    

This code creates an array of typed User objects, each with an id and name, for n users.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop creating and adding User objects to the array.
• How many times: Exactly n times, once for each user.

How Execution Grows With Input

As n grows, the number of User objects created and added grows directly with n.

Input Size (n)	Approx. Operations
10	About 10 User creations and insertions
100	About 100 User creations and insertions
1000	About 1000 User creations and insertions

Pattern observation: The work grows evenly and directly as n increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to create and store users grows in a straight line with the number of users.

Common Mistake

[X] Wrong: "Typed classes make the code slower because of extra type checks at runtime."

[OK] Correct: TypeScript types are removed when the code runs, so typed classes do not add runtime cost. The time depends on how many objects you create, not the types.

Interview Connect

Understanding how typed classes affect time helps you explain how your code scales and why types are safe without slowing down your program.

Self-Check

"What if we added a nested loop inside the createUsers function to assign friends to each user? How would the time complexity change?"

Practice

1. Why are typed classes important in TypeScript?

easy

A. They make the code run faster at runtime.

B. They allow writing code without any rules.

C. They help catch errors before running the code.

D. They remove the need for functions.

Solution

1. Step 1: Understand typed classes role

   Typed classes define what kind of data each property should hold, helping catch mistakes early.

2. Step 2: Identify the benefit

   This early error detection happens before running the program, improving code safety.

3. Final Answer:

   They help catch errors before running the code. -> Option C

4. Quick Check:

   Typed classes catch errors early = B [OK]

Hint: Typed classes catch mistakes early, not speed up runtime [OK]

Common Mistakes:

• Thinking typed classes speed up code execution
• Believing typed classes remove the need for functions
• Assuming typed classes allow ignoring data rules

2. Which of the following is the correct way to declare a typed class property in TypeScript?

easy

A. class User { name = string; }

B. class User { name: string; }

C. class User { string name; }

D. class User { name -> string; }

Solution

1. Step 1: Recall TypeScript property syntax

   In TypeScript, properties are declared with a colon between name and type, like name: string;.

2. Step 2: Check each option

   class User { name: string; } uses correct syntax. Options A, B, and C use invalid syntax.

3. Final Answer:

   class User { name: string; } -> Option B

4. Quick Check:

   Property declaration uses colon = D [OK]

Hint: Use colon between property and type in classes [OK]

Common Mistakes:

• Using equals sign instead of colon for type
• Placing type before property name
• Using arrows or other symbols instead of colon

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

class Product {
  name: string;
  price: number;
  constructor(name: string, price: number) {
    this.name = name;
    this.price = price;
  }
  display() {
    return `${this.name} costs $${this.price}`;
  }
}
const item = new Product('Book', 15);
console.log(item.display());

medium

A. Book costs $15

B. Book costs $15.00

C. undefined costs $15

D. Error: price is not a string

Solution

1. Step 1: Understand class and constructor

   The constructor sets name to 'Book' and price to 15.

2. Step 2: Check display method output

   The display method returns a string combining name and price exactly as "Book costs $15".

3. Final Answer:

   Book costs $15 -> Option A

4. Quick Check:

   Display method returns formatted string = C [OK]

Hint: Check constructor values and string template output [OK]

Common Mistakes:

• Expecting price to show decimals automatically
• Confusing undefined property values
• Thinking type mismatch causes runtime error here

4. Identify the error in this TypeScript class and fix it:

class Person {
  age: number;
  constructor(age: string) {
    this.age = age;
  }
}

medium

A. Change constructor parameter type to number

B. Change property type to string

C. Remove type annotations completely

D. No error, code is correct

Solution

1. Step 1: Compare property and constructor types

   The property age is a number, but constructor expects a string parameter.

2. Step 2: Fix type mismatch

   Change constructor parameter type to number to match property type.

3. Final Answer:

   Change constructor parameter type to number -> Option A

4. Quick Check:

   Property and parameter types must match = A [OK]

Hint: Match constructor parameter types with property types [OK]

Common Mistakes:

• Ignoring type mismatch between property and parameter
• Changing property type unnecessarily
• Assuming no error when types differ

5. You want to create a typed class in TypeScript that holds a list of users with their names and ages. Which approach best ensures type safety and easy maintenance?

hard

A. Use a class with properties typed as strings only.

B. Use a class with any type for properties to allow flexibility.

C. Use plain JavaScript objects without types for faster coding.

D. Use a class with typed properties and an array of user objects typed with an interface.

Solution

1. Step 1: Consider type safety and maintenance

   Typed properties and interfaces help catch errors and make code clear for teamwork.

2. Step 2: Evaluate options

   A class with typed properties and an array of user objects typed with an interface ensures safety and clarity. Other options sacrifice safety or clarity.

3. Final Answer:

   Use a class with typed properties and an array of user objects typed with an interface. -> Option D

4. Quick Check:

   Typed classes + interfaces = best practice [OK]

Hint: Combine typed classes with interfaces for clear, safe code [OK]

Common Mistakes:

• Using any type loses type safety
• Ignoring interfaces reduces code clarity
• Typing all properties as strings limits data accuracy