Constructor parameter types in Typescript - Time & Space Complexity

Time Complexity: Constructor parameter types

O(n)

Understanding Time Complexity

Let's see how the time it takes to run code changes when we use constructor parameters in TypeScript classes.

We want to know how the number of operations grows as we create more objects with parameters.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Person {
  name: string;
  age: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}

const people: Person[] = [];
for (let i = 0; i < n; i++) {
  people.push(new Person(`Name${i}`, i));
}
    

This code creates a list of Person objects, each with a name and age passed to the constructor.

Identify Repeating Operations

• Primary operation: Creating a new Person object inside a loop.
• How many times: The loop runs n times, so the constructor runs n times.

How Execution Grows With Input

Each time we add one more person, we do one more constructor call and one more push to the array.

Input Size (n)	Approx. Operations
10	About 10 constructor calls and 10 pushes
100	About 100 constructor calls and 100 pushes
1000	About 1000 constructor calls and 1000 pushes

Pattern observation: The number of operations grows directly with n, so doubling n doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to create all objects grows in a straight line as we add more items.

Common Mistake

[X] Wrong: "The constructor runs only once, so the time is constant regardless of n."

[OK] Correct: The constructor runs every time we create a new object, so it runs n times when we create n objects.

Interview Connect

Understanding how constructors affect performance helps you explain object creation costs clearly and confidently in real projects.

Self-Check

"What if the constructor did some extra work inside a loop for each object? How would the time complexity change?"

Practice

1. What does specifying parameter types in a TypeScript class constructor do?

easy

A. It changes the class name based on parameters.

B. It runs the constructor faster at runtime.

C. It automatically creates getter and setter methods.

D. It tells what kind of values the constructor expects when creating an object.

Solution

1. Step 1: Understand constructor parameter types

   Constructor parameter types define what type of values the constructor expects to receive when creating an instance of the class.

2. Step 2: Identify the purpose of parameter types

   They help TypeScript check for errors before running the code by ensuring the right types are passed.

3. Final Answer:

   It tells what kind of values the constructor expects when creating an object. -> Option D

4. Quick Check:

   Constructor parameter types = Define expected input types [OK]

Hint: Constructor types define expected inputs clearly [OK]

Common Mistakes:

• Thinking parameter types speed up runtime
• Confusing parameter types with method creation
• Believing parameter types rename classes

2. Which of the following is the correct way to declare a constructor with typed parameters in TypeScript?

easy

A. constructor(name: string, age: number) {}

B. constructor(name string, age number) {}

C. constructor(name, age) : string, number {}

D. constructor(name => string, age => number) {}

Solution

1. Step 1: Review TypeScript constructor syntax

   In TypeScript, parameter types are declared after the parameter name with a colon, like name: string.

2. Step 2: Check each option

   constructor(name: string, age: number) {} uses correct syntax with colons and types. Options A, C, and D use invalid syntax.

3. Final Answer:

   constructor(name: string, age: number) {} -> Option A

4. Quick Check:

   Parameter types use colon syntax [OK]

Hint: Use colon after parameter name for types [OK]

Common Mistakes:

• Omitting colons between name and type
• Using commas or arrows instead of colons
• Placing types after parentheses

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

class Person {
  constructor(public name: string, private age: number) {}
  getAge() { return this.age; }
}
const p = new Person('Alice', 30);
console.log(p.name);
console.log(p.getAge());

medium

A. Error: age is private

B. Alice
30

C. Alice
undefined

D. undefined
30

Solution

1. Step 1: Understand public and private in constructor

   The public name creates a public property accessible outside. The private age creates a private property accessible only inside the class.

2. Step 2: Check console.log outputs

   p.name prints 'Alice' because it's public. p.getAge() returns 30 because the method accesses private age internally.

3. Final Answer:

   Alice 30 -> Option B

4. Quick Check:

   Public accessible, private via method [OK]

Hint: Public props accessible directly, private via methods [OK]

Common Mistakes:

• Expecting private properties accessible outside
• Confusing undefined with private access
• Thinking private causes runtime error

4. Identify the error in this TypeScript class constructor:

class Car {
  constructor(make: string, year: number) {
    this.make = make;
    this.year = year;
  }
}

medium

A. Constructor must have return type declared

B. Cannot assign parameters to 'this' inside constructor

C. Class properties 'make' and 'year' are missing

D. Missing type for parameter 'year' in constructor

Solution

1. Step 1: Check class properties and constructor assignments

2. Step 2: Check class properties

   The class does not declare properties 'make' and 'year' before assigning them in the constructor, which causes an error: Property 'make' does not exist on type 'Car'.

3. Final Answer:

   Class properties 'make' and 'year' are missing -> Option C

4. Quick Check:

   Declare class properties before use [OK]

Hint: Declare properties before assigning in constructor [OK]

Common Mistakes:

• Ignoring missing property declarations
• Thinking constructor parameters auto-create properties
• Expecting return type in constructor

5. You want to create a class Book that takes a title (string), pages (number), and an optional author (string). Which constructor declaration correctly types these parameters and allows creating a Book by omitting the author argument (leaving author as undefined)?

hard

A. constructor(title: string, pages: number, author?: string) {}

B. constructor(title: string, pages: number, author: string = '') {}

C. constructor(title: string, pages: number, author: string) {}

D. constructor(title: string, pages: number, author?: string = '') {}

Solution

1. Step 1: Understand optional parameters in TypeScript

   Optional parameters are marked with a question mark ? after the parameter name, allowing them to be omitted when calling the constructor, leaving the parameter undefined.

2. Step 2: Evaluate each option

   constructor(title: string, pages: number, author?: string = '') {}: author?: string = '' -- omitting sets to '', not undefined. constructor(title: string, pages: number, author: string = '') {}: author: string = '' -- omitting sets to ''. constructor(title: string, pages: number, author: string) {} requires author. constructor(title: string, pages: number, author?: string) {}: author?: string -- omitting leaves undefined.

3. Final Answer:

   constructor(title: string, pages: number, author?: string) {} -> Option A

4. Quick Check:

   Optional params use ? after name [OK]

Hint: Use ? after parameter name for optional [OK]

Common Mistakes:

• Using default value without ? for optional
• Forgetting to mark optional parameters
• Mixing optional and default syntax incorrectly