what is the difference and what is better to use: Factory Function vs Constructor Function. Please reason your approach, so I can learn

Question

I have two functions that do the same thing (I may be wrong, but seems like they do the same thing in my newbie opinion):

// Factory Function
function createCircle(radius) {
    return {
        radius,
        draw: function() {
            console.log('draw');
        }
    }
}
const newCircle = createCircle(2);
console.log(newCircle);

And

// Constructor Function
function Circle(radius) {
    this.radius = radius;
    this.draw = function() {
        console.log('draw')
    }
}
const another = new Circle(2);
console.log(another);

Are there some pros and cons of these approaches? Just seeking for the opinions of more experienced developers.

"in your opinion" - SO is not a good place to ask opinion-based questions — Jan Stránský, Aug 13 '20 at 09:58
Remove "opinion" from the question and it becomes "what is the difference between these two things", which I think is perfectly reasonable. — Gorse horse, Aug 13 '20 at 10:07

score 1 · Accepted Answer · answered Aug 13 '20 at 10:02

For most circumstances I'd say the constructor function is better.

It lets you check whether the object is a Circle by using the instanceof operator (i.e. circle instanceof Circle)
It makes the code more readable - when someone sees new Circle they know that a class is being instantiated, and they know that it will have a predictable behaviour, whereas for the first case they need to check the function definition (or documentation) of createCircle before they know anything about what this function is returning.
You can optimise the constructor a little more - if you define the draw function on the class's prototype (i.e. Circle.prototype.draw = function(){/*etc*/}) then it doesn't get declared each time you call new Circle.

That said, if you're creating an object with just a few properties and no methods then using the first example is perfectly reasonable.

what is the difference and what is better to use: Factory Function vs Constructor Function. Please reason your approach, so I can learn

1 Answers1