Constructor function vs Factory functions

Question

Can someone clarify the difference between a constructor function and a factory function in Javascript.

When to use one instead of the other?

Answer 1

A Constructor function example

function User(name) {
  this.name = name;
  this.isAdmin = false;
}

let user = new User("Jack");

• new creates an object prototyped on User.prototype and calls User with the created object as its this value.

• new treats an argument expression for its operand as optional:

       let user = new User;
  

  would cause new to call User with no arguments.

• new returns the object it created, unless the constructor returns an object value, which is returned instead. This is an edge case which for the most part can be ignored.

Pros and Cons

Objects created by constructor functions inherit properties from the constructor's prototype property, and return true using the instanceOf operator on the constructor function.

The above behaviors can fail if you dynamically change the value of the constructor's prototype property after having already used the constructor. Doing so is rare, and it can't be changed if the constructor were created using the class keyword.

Constructor functions can be extended using the extends keyword.

Constructor functions can't return null as an error value. Since it's not an object data type, it is ignored by new.

A Factory function example

function User(name, age) {
  return {
    name,
    age,
  }
};

let user = User("Tom", 23);

Here the factory function is called without new. The function is entirely responsible for the direct or indirect use if its arguments and the type of object it returns. In this example it returns a simple [Object object] with some properties set from arguments.

Pros and Cons

Easily hides the implementation complexities of object creation from the caller. This is particularly useful for native code functions in a browser.

The factory function need not always return objects of the same type, and could even return null as an error indicator.

In simple cases, factory functions can be simple in structure and meaning.

Objects returned do not generally inherit from the factory function's prototype property, and return false from instanceOf factoryFunction.

The factory function can't be safely extended using the extends keyword because extended objects would inherit from the factory functions prototype property instead of from the prototype property of the constructor used by the factory function.