Constructors in classes in Javascript - Time & Space Complexity

Time Complexity: Constructors in classes

O(n)

Understanding Time Complexity

Let's see how the time it takes to run a constructor in a class changes as we create more objects.

We want to know how the work done inside the constructor grows when we make many instances.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

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

This code creates n Person objects, each with a name and age set by the constructor.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for loop that creates n Person objects.
• How many times: The constructor runs once for each of the n iterations.

How Execution Grows With Input

Each new object takes a small fixed amount of work to set properties.

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

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 you add more objects.

Common Mistake

[X] Wrong: "The constructor runs only once no matter how many objects are created."

[OK] Correct: Each object needs its own constructor call to set up its properties, so the constructor runs once per object.

Interview Connect

Understanding how constructors scale helps you explain object creation costs clearly and confidently in real coding situations.

Self-Check

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

Practice

1. What is the main purpose of a constructor in a JavaScript class?

easy

A. To create static properties for the class

B. To define methods that all objects share

C. To initialize new objects with default or given values

D. To delete objects when they are no longer needed

Solution

1. Step 1: Understand the role of a constructor

   A constructor is a special method in a class that runs automatically when a new object is created.

2. Step 2: Identify what constructors do

   Constructors set initial values for the object's properties using this.

3. Final Answer:

   To initialize new objects with default or given values -> Option C

4. Quick Check:

   Constructor = initialize object [OK]

Hint: Constructors always set up new objects [OK]

Common Mistakes:

• Confusing constructors with regular methods
• Thinking constructors delete objects
• Believing constructors create static properties

2. Which of the following is the correct syntax to define a constructor inside a JavaScript class?

easy

A. constructor() { this.name = 'John'; }

B. function constructor() { this.name = 'John'; }

C. def constructor() { this.name = 'John'; }

D. constructor = () => { this.name = 'John'; }

Solution

1. Step 1: Recall JavaScript class constructor syntax

   In JavaScript classes, the constructor is defined as a method named constructor without the function keyword.

2. Step 2: Check each option

   constructor() { this.name = 'John'; } uses the correct syntax: constructor() { ... }. Options B and C incorrectly use keywords from other languages or function syntax. constructor = () => { this.name = 'John'; } uses arrow function syntax, which is invalid for constructors.

3. Final Answer:

   constructor() { this.name = 'John'; } -> Option A

4. Quick Check:

   Constructor method = constructor() { } [OK]

Hint: Constructor is a method named exactly 'constructor' [OK]

Common Mistakes:

• Adding 'function' keyword before constructor
• Using arrow functions for constructors
• Using other language syntax like 'def'

3. What will be the output of the following code?

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    return `Hello, ${this.name}!`;
  }
}
const p = new Person('Alice');
console.log(p.greet());

medium

A. Hello, undefined!

B. Hello, Alice!

C. Hello, !

D. TypeError: p.greet is not a function

Solution

1. Step 1: Understand object creation with constructor

   The Person class constructor sets this.name to the passed argument. Here, 'Alice' is passed, so p.name becomes 'Alice'.

2. Step 2: Evaluate the greet method output

   The greet method returns a string using this.name, which is 'Alice'. So, p.greet() returns 'Hello, Alice!'.

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   Constructor sets name, greet uses it = Hello, Alice! [OK]

Hint: Constructor sets property, method uses it [OK]

Common Mistakes:

• Forgetting to pass argument to constructor
• Expecting undefined because of missing this
• Thinking greet is not a method

4. Identify the error in this class definition:

class Car {
  constructor(make, model) {
    make = make;
    model = model;
  }
}

medium

A. Properties should be assigned using this, like this.make = make;

B. The class name should be lowercase

C. The constructor should not have parameters

D. The constructor must return an object explicitly

Solution

1. Step 1: Review property assignment in constructor

   Inside a constructor, to assign values to the object's properties, you must use this.propertyName. Simply writing make = make; assigns the parameter to itself, which does nothing.

2. Step 2: Identify the correct fix

   Using this.make = make; and this.model = model; correctly assigns the values to the new object's properties.

3. Final Answer:

   Properties should be assigned using this, like this.make = make; -> Option A

4. Quick Check:

   Use this.property = parameter [OK]

Hint: Always use this.property to assign inside constructor [OK]

Common Mistakes:

• Assigning parameters to themselves
• Not using this to set properties
• Thinking constructor must return explicitly

5. You want to create a Book class where each book has a title and an optional author. If no author is given, it should default to 'Unknown'. Which constructor correctly implements this behavior?

hard

A. constructor(title, author) { this.title = title; if (author) this.author = author; }

B. constructor(title, author) { this.title = title; this.author = author || 'Unknown'; }

C. constructor(title) { this.title = title; this.author = 'Unknown'; }

D. constructor(title, author = 'Unknown') { this.title = title; this.author = author; }

Solution

1. Step 1: Understand optional parameters in constructors

   Use default parameter values for optional arguments. constructor(title, author = 'Unknown') { this.title = title; this.author = author; }: constructor(title, author = 'Unknown') sets author to 'Unknown' if not provided or undefined, but keeps passed falsy values like empty string.

2. Step 2: Check other options

   B uses ||, which defaults falsy values (not just missing). C has no author param. D sets only if truthy, else undefined.

3. Final Answer:

   constructor(title, author = 'Unknown') { this.title = title; this.author = author; } -> Option D

4. Quick Check:

   Optional param: param = default [OK]

Hint: Default params for optional constructor args [OK]

Common Mistakes:

• Using logical OR (||) which treats falsy as missing
• Omitting the optional parameter entirely
• Not providing a default, leaving undefined