why to use prototypes over methods

Question

I was watching tutorial about functions. And there is an example

function Car(){
this.running = false
}
Car.prototype.start = function(){
    this.running = true
}
Car.prototype.stop = function(){
    this.running = false
}

var c = new Car()
c.running // false
c.start // true
c.running // true

but there is another way of doing the same thing

function Car(){
    this.running = false;
    this.start = function(){
        this.running = true
    }
    this.stop = function(){
        this.running = false
    }
}

var c = new Car()
c.running // false
c.start // true
c.running // true

Question:

1. what is pros and cons of using prototypes compared to methods?
2. when should we avoid/use prototypes?
3. when should we avoid/use methods?

Answer 1

Ideally, whenever you want to share properties among multiple instances of a type, you make use of the prototype. In your first example

function Car() {
  this.running = false
}
Car.prototype.start = function() {
  this.running = true
}
Car.prototype.stop = function() {
  this.running = false
}

var c = new Car()
var d = new Car()
var e = new Car()
console.log(c.__proto__ === d.__proto__) //true
c.running // false
c.start // true
c.running // true

Here prototype object will remain same for c, d and e, Hence you are saving runtime memory consumption as you are reusing same functions for all the three, but with different context.

In your second example

function Car(){
    this.running = false;
    this.start = function(){
        this.running = true
    }
    this.stop = function(){
        this.running = false
    }
}

var c = new Car() // check comparing  c.__proto__ ===  d.__proto__
var d =  new Car()
var e = new Car()
c.running // false
c.start // true
c.running // 

Here c, d, e will have their own copy of function, Consuming more memory!! Ideally, one should think about reusability while designing such objects. prototype way seems to better and performant than method way.