Constructor functions in Javascript - Time & Space Complexity

Time Complexity: Constructor functions

O(n)

Understanding Time Complexity

Let's see how the time it takes to create objects with constructor functions changes as we make more objects.

We want to know how the work grows when making many objects using a constructor.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function Person(name, age) {
  this.name = name;
  this.age = age;
  this.greet = function() {
    console.log(`Hi, I am ${this.name}`);
  };
}

const people = [];
for (let i = 0; i < n; i++) {
  people.push(new Person(`Person${i}`, 20 + i));
}
    

This code creates n person objects using a constructor function and stores them in an array.

Identify Repeating Operations

• Primary operation: Creating a new Person object inside the 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, the work grows by one constructor call.

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

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

Final Time Complexity

Time Complexity: O(n)

This means if you double the number of objects, the time to create them roughly doubles too.

Common Mistake

[X] Wrong: "Creating many objects with a constructor is instant and does not depend on how many objects we make."

[OK] Correct: Each object needs time to be created, so more objects mean more time spent.

Interview Connect

Understanding how object creation scales helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

"What if the constructor function included a loop inside it? How would that affect the time complexity when creating n objects?"

Practice

1. What does the new keyword do when used with a constructor function in JavaScript?

easy

A. Creates a new object and sets this to that object inside the constructor

B. Calls the constructor function without creating a new object

C. Deletes the constructor function after running

D. Returns a string representation of the constructor

Solution

1. Step 1: Understand the role of new

   The new keyword creates a fresh object and sets this inside the constructor to that object.

2. Step 2: Effect on constructor function

   It allows the constructor to initialize properties on the new object and then returns it automatically.

3. Final Answer:

   Creates a new object and sets this to that object inside the constructor -> Option A

4. Quick Check:

   new creates object = A [OK]

Hint: Remember: new makes a new object and sets this [OK]

Common Mistakes:

• Thinking new just calls the function without creating an object
• Believing new returns a string
• Confusing new with deleting functions

2. Which of the following is the correct syntax to define a constructor function named Car?

easy

A. function Car() { this.make = 'Toyota'; }

B. var Car = () => { this.make = 'Toyota'; }

C. class Car { make = 'Toyota'; }

D. function Car[] { this.make = 'Toyota'; }

Solution

1. Step 1: Identify valid constructor function syntax

   A constructor function is a normal function with a capitalized name, like function Car() {}.

2. Step 2: Check each option

   function Car() { this.make = 'Toyota'; } uses correct function syntax. var Car = () => { this.make = 'Toyota'; } uses arrow function which does not bind this properly. class Car { make = 'Toyota'; } is a class, not a constructor function. function Car[] { this.make = 'Toyota'; } has invalid syntax with square brackets.

3. Final Answer:

   function Car() { this.make = 'Toyota'; } -> Option A

4. Quick Check:

   Constructor function = normal function with capital name [OK]

Hint: Use normal function with capital name for constructor [OK]

Common Mistakes:

• Using arrow functions as constructors
• Trying to use class syntax instead of function
• Writing invalid function syntax with brackets

3. What will be the output of this code?

function Person(name) {
  this.name = name;
}
const p = new Person('Alice');
console.log(p.name);

medium

A. undefined

B. Error

C. Alice

D. Person

Solution

1. Step 1: Understand object creation with constructor

   The new Person('Alice') creates a new object and sets this.name to 'Alice'.

2. Step 2: Access the property

   Logging p.name prints the value assigned, which is 'Alice'.

3. Final Answer:

   Alice -> Option C

4. Quick Check:

   Property set by constructor = 'Alice' [OK]

Hint: Remember: this.name sets property on new object [OK]

Common Mistakes:

• Expecting undefined because of missing return
• Confusing constructor name with property value
• Thinking console.log prints function name

4. Find the error in this constructor function:

function Animal(type) {
  type = type;
  this.type = type;
}
const a = new Animal('Dog');
console.log(a.type);

medium

A. Missing new keyword when creating object

B. Cannot assign this.type inside constructor

C. Constructor function name should be lowercase

D. The assignment type = type; does nothing and is redundant

Solution

1. Step 1: Analyze the assignment type = type;

   This line assigns the parameter type to itself, which does nothing and is unnecessary.

2. Step 2: Check other parts

   The line this.type = type; correctly assigns the parameter to the object property. The new keyword is used correctly. Function name capitalization is fine.

3. Final Answer:

   The assignment type = type; does nothing and is redundant -> Option D

4. Quick Check:

   Redundant self-assignment = B [OK]

Hint: Check if variables are assigned to themselves unnecessarily [OK]

Common Mistakes:

• Thinking type = type; sets object property
• Forgetting to use new keyword
• Believing constructor names must be lowercase

5. You want to create a constructor function Book that takes title and author as parameters and adds a method describe that returns a string like "Title by Author". Which code correctly does this?

hard

A. function Book(title, author) {
this.title = title;
this.author = author;
}
Book.describe = function() {
return `${this.title} by ${this.author}`;
};

B. function Book(title, author) {
this.title = title;
this.author = author;
this.describe = function() {
return `${this.title} by ${this.author}`;
};
}

C. function Book(title, author) {
this.title = title;
this.author = author;
describe() {
return `${this.title} by ${this.author}`;
}
}

D. function Book(title, author) {
this.title = title;
this.author = author;
this.describe = `${this.title} by ${this.author}`;
}

Solution

1. Step 1: Check method definition inside constructor

   function Book(title, author) { this.title = title; this.author = author; this.describe = function() { return `${this.title} by ${this.author}`; }; } correctly defines describe as a function property on this, so each object has its own method.

2. Step 2: Analyze other options

   function Book(title, author) { this.title = title; this.author = author; } Book.describe = function() { return `${this.title} by ${this.author}`; }; adds describe to the constructor function itself, not instances. function Book(title, author) { this.title = title; this.author = author; describe() { return `${this.title} by ${this.author}`; } } has invalid syntax for method inside function. function Book(title, author) { this.title = title; this.author = author; this.describe = `${this.title} by ${this.author}`; } assigns a string, not a function, so describe is not callable.

3. Final Answer:

   function Book(title, author) { this.title = title; this.author = author; this.describe = function() { return `${this.title} by ${this.author}`; }; } -> Option B

4. Quick Check:

   Method as function property on this = A [OK]

Hint: Define methods as functions on this inside constructor [OK]

Common Mistakes:

• Adding methods to constructor function instead of instances
• Using invalid syntax for methods inside functions
• Assigning method as string instead of function