Class property declarations in Typescript - Time & Space Complexity

Time Complexity: Class property declarations

O(n)

Understanding Time Complexity

Let's see how the time it takes to run code changes when we use class property declarations in TypeScript.

We want to know how declaring properties affects the work done when creating objects.

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 n Person objects, each with two properties set during construction.

Identify Repeating Operations

• Primary operation: Creating a new Person object and assigning its properties inside the constructor.
• How many times: This happens once for each of the n iterations in the loop.

How Execution Grows With Input

Each new object requires setting two properties, and this happens for every item in the loop.

Input Size (n)	Approx. Operations
10	About 20 property assignments (2 per object x 10 objects)
100	About 200 property assignments
1000	About 2000 property assignments

Pattern observation: The work grows directly with the number of objects created.

Final Time Complexity

Time Complexity: O(n)

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

Common Mistake

[X] Wrong: "Declaring properties inside the class makes object creation instant or constant time regardless of how many objects."

[OK] Correct: Each object still needs its own properties set, so the time grows with the number of objects created.

Interview Connect

Understanding how object creation scales helps you explain performance when working with many instances in real projects.

Self-Check

What if we added a loop inside the constructor that runs m times? How would the time complexity change?

Practice

1. What is the purpose of declaring properties inside a TypeScript class?

easy

A. To import external modules into the class

B. To define functions that the class can use

C. To create new classes inside the current class

D. To store data related to each object created from the class

Solution

1. Step 1: Understand what class properties represent

   Class properties hold data or information specific to each object created from the class.

2. Step 2: Differentiate properties from methods and other elements

   Functions inside classes are called methods, not properties. Properties are for storing values.

3. Final Answer:

   To store data related to each object created from the class -> Option D

4. Quick Check:

   Class properties = store object data [OK]

Hint: Properties hold object data, methods hold actions [OK]

Common Mistakes:

• Confusing properties with methods
• Thinking properties create new classes
• Believing properties import modules

2. Which of the following is the correct way to declare a class property with a type and default value in TypeScript?

easy

A. class Car { speed: = 0 number; }

B. class Car { speed: number = 0; }

C. class Car { number speed = 0; }

D. class Car { speed = number 0; }

Solution

1. Step 1: Recall TypeScript property syntax

   Properties are declared as name: type = defaultValue;.

2. Step 2: Check each option for correct syntax

   class Car { speed: number = 0; } matches the correct syntax: speed: number = 0;. Others have incorrect order or missing punctuation.

3. Final Answer:

   class Car { speed: number = 0; } -> Option B

4. Quick Check:

   Property syntax = name: type = value [OK]

Hint: Use colon for type, equals for default value [OK]

Common Mistakes:

• Swapping type and value order
• Missing colon between name and type
• Using wrong punctuation

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

class Person {
  name: string = "Alice";
  age: number = 30;
}
const p = new Person();
console.log(p.name + ' is ' + p.age + ' years old');

medium

A. "Alice is 30 years old"

B. "undefined is undefined years old"

C. Error: Property 'name' does not exist

D. "Alice is undefined years old"

Solution

1. Step 1: Understand property initialization

   The class Person has properties name and age initialized with "Alice" and 30 respectively.

2. Step 2: Check the output of console.log

   Creating an object p sets these properties. The log concatenates them into the string "Alice is 30 years old".

3. Final Answer:

   "Alice is 30 years old" -> Option A

4. Quick Check:

   Initialized properties print correct values [OK]

Hint: Default values print as expected when object created [OK]

Common Mistakes:

• Assuming properties are undefined without constructor
• Confusing property names or types
• Expecting runtime errors without syntax mistakes

4. Identify the error in this TypeScript class property declaration:

class Book {
  title string = "Unknown";
  pages: number = 100;
}

medium

A. Class name should be lowercase

B. Missing default value for 'pages' property

C. Missing colon ':' between property name and type in 'title string = "Unknown";'

D. Properties cannot have default values

Solution

1. Step 1: Check syntax of property declarations

   Property declarations require a colon ':' between the name and type, like title: string.

2. Step 2: Identify the incorrect line

   The line title string = "Unknown"; misses the colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' between property name and type in 'title string = "Unknown";' -> Option C

4. Quick Check:

   Property syntax requires colon ':' [OK]

Hint: Always put ':' between property name and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Thinking default values are not allowed
• Ignoring case sensitivity of class names (not an error)

5. You want to create a TypeScript class Student with a property grades that stores an array of numbers and defaults to an empty array. Which is the correct declaration to ensure each object has its own separate array?

hard

A. class Student { grades: number[] = []; }

B. class Student { static grades: number[] = []; }

C. class Student { grades: number[]; } // no default value

D. class Student { grades = []; }

Solution

1. Step 1: Understand instance vs static properties

   Instance properties belong to each object separately; static properties are shared by all objects.

2. Step 2: Check which option creates separate arrays per object

   class Student { grades: number[] = []; } declares grades as an instance property with default empty array, so each object gets its own array.

3. Step 3: Analyze other options

   class Student { static grades: number[] = []; } uses static, so all objects share one array (not desired). class Student { grades: number[]; } // no default value has no default, so array is undefined unless set later. class Student { grades = []; } misses type annotation explicitly, inferring any[] instead of number[].

4. Final Answer:

   class Student { grades: number[] = []; } -> Option A

5. Quick Check:

   Instance property with default array = separate arrays [OK]

Hint: Use instance property with default [] for separate arrays [OK]

Common Mistakes:

• Using static for properties needing separate values
• Not initializing arrays causing undefined errors
• Omitting type annotation (less clear)