🏭 JavaScript Functions Deep Dive #7: Factory Function vs Constructor Function

In JavaScript, there are two common patterns for creating multiple similar objects:

1. Factory Functions

2. Constructor Functions

Both help in object creation, but they differ in syntax, style, behavior, and flexibility.

In this article, we’ll explore:

• ✅ What factory and constructor functions are

• ✅ Syntax differences and best practices

• ✅ Pros and cons of each

• ✅ Real-world usage examples

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🧪 Why Use These Patterns?

JavaScript doesn’t have traditional classes (before ES6). Instead, we use functions to create objects — either:

• Factory functions (normal functions that return objects), or

• Constructor functions (functions used with the new keyword)

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🏭 1. Factory Function — A Function That Returns an Object

🔹 Syntax:

function createUser(name, age) {
  return {
    name,
    age,
    greet() {
      return `Hi, I'm ${this.name}`;
    },
  };
}

const user1 = createUser("Alice", 25);
console.log(user1.greet()); // Hi, I'm Alice

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✅ Key Characteristics

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🔐 Private Variables Example with Closure

function createCounter() {
  let count = 0;
  return {
    increment() {
      count++;
      return count;
    },
    getCount() {
      return count;
    },
  };
}

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.getCount());  // 1

✅ count is truly private — inaccessible from outside.

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👷‍♂️ 2. Constructor Function — Uses this and new

🔹 Syntax:

function User(name, age) {
  this.name = name;
  this.age = age;
  this.greet = function () {
    return `Hello, I'm ${this.name}`;
  };
}

const user2 = new User("Bob", 30);
console.log(user2.greet()); // Hello, I'm Bob

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✅ Key Characteristics

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🧠 Behind the Scenes of new

When you call a constructor with new:

1. A new object {} is created.

2. this inside the function refers to that object.

3. Properties are attached to this.

4. The object is returned automatically.

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💡 When to Use What?

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🧑‍💻 Real-World Example

🔹 Factory Example

function createProduct(name, price) {
  return {
    name,
    price,
    discount(percent) {
      return price * (1 - percent / 100);
    },
  };
}

🔹 Constructor Example

function Product(name, price) {
  this.name = name;
  this.price = price;
}

Product.prototype.discount = function (percent) {
  return this.price * (1 - percent / 100);
};

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🔚 Summary

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✅ Final Thoughts

Both factory and constructor functions are valid ways to create objects in JavaScript. The best choice depends on:

• Your team’s preference (OOP vs functional)

• Performance requirements

• Code readability and privacy needs

In modern JavaScript, factory functions are often favored due to their simplicity and composability — especially in React, testing utilities, and pure functions.