Using Prototypes Over Classes In JavaScript

Question

Is there a reason to use prototypes over classes? If I understand correctly, prototypes would be more efficient if I had defined functions in the constructor (but that is not the case here). Is the only thing different between these implementations syntax?

    class Quiz {
    constructor(questions) {
        this.score = 0;
        this.questionArray = questions;
        this.questionIndex = 0;
    }

    getCurrentQuestionObj() {
        return this.questionArray[this.questionIndex];
    }

    isGameOver() {
        return this.questionArray.length === this.questionIndex;
    }

    guess(answer) {
        if(this.getCurrentQuestionObj().isCorrectAnswer(answer)) {
            this.score++;
        }
        this.questionIndex++;
    }
}

-

function Quiz(questions) {
    this.score = 0;
    this.questions = questions;
    this.questionIndex = 0;
}

Quiz.prototype.getCurrentQuestionObj = function() {
    return this.questions[this.questionIndex];
}

Quiz.prototype.isGameOver = function() {
    return this.questions.length === this.questionIndex;
}

Quiz.prototype.guess = function(answer) {
    if(this.getCurrentQuestionObj().correctAnswer(answer)) {
        this.score++;
    }
    this.questionIndex++;
}

john_omalley · Accepted Answer · 2017-02-09 01:58:41Z

2

ES6 classes are nothing more than sugar. Your two examples are equivalent.

Regarding functions declared in the constructor - you're right that those would be slightly less efficient. If you set 'this.foo = function() {}' in the constructor a new function is created every time you instantiate using the constructor.

answered Feb 9, 2017 at 1:58

john_omalley

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8 Comments

MaxZoom Over a year ago

But when using a prototype you can broader the class definition, while class notation is not so easily expandable.

Eric Larson Over a year ago

@john_omalley Is there any downsides of using classes? I know it's a more recent feature that came with ES6, but I don't see it used too often.

john_omalley Over a year ago

The only downside I can think of is that you need to transpile them to work in older browsers.

john_omalley Over a year ago

re: 'class notation is not so easily expandable'... not sure what you mean but it's really 100% equivalent. try it out in Node or Chrome.

Eric Larson Over a year ago

I understand using prototype would allow me to expand on the second implementation, but how would I expand on the class definition?

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