Accessing Javascript object prototype [duplicate]

Question

As I understand, in JavaScript every object has a prototype and it exposes some default properties. I have the following code where I'm trying to set the Year property of two objects through prototype. But both the calls are failing.

How can I override toLocalString() for any object if I don't have access to the prototype? Please note that the following code is to test on the prototype property, but my intention is to override the toLocalString() method.

var car = {
    Make: 'Nissan',
    Model: 'Altima'
};

car.Year = 2014;
alert(car.Year);

alert(car.prototype); // returns undefined

car.prototype.Year = 2014; // Javascript error

// --------------

function Car() {
    this.Make = 'NISSAN';
    this.Model = 'Atlanta';
}

var v = new Car();
v.prototype.Year = 2014; // JavaScript error
alert(v.prototype);

In your bottom example, it should be Car.prototype.Year = 2014 - you set the prototype on the object function - not the created instance. — tymeJV
– tymeJV, Commented Oct 1, 2014 at 19:49
Do you mean toLocaleString() instead of toLocalString() or do you want to implement your own method toLocalString()? — keenthinker
– keenthinker, Commented Oct 1, 2014 at 20:02
The prototype property belongs on a function. The function uses that prototype property when called with new to construct instance objects. Setting prototype on a non-function object has no effect. — apsillers
– apsillers, Commented Oct 1, 2014 at 20:15

Ben Aston · Accepted Answer · 2015-05-31 20:54:33Z

You do have access to the prototype property, but it is only present on Functions.

var car = {
    Make: 'Nissan',
    Model: 'Altima'
};

This is the same as:

var car = new Object();
car.Make = 'Nissan';
car.Model = 'Altima';

So, car.__proto__ === Object.prototype.

And car.prototype === undefined as the prototype property is only present on Functions (as I already said).

function Car() {
    this.Make = 'NISSAN';
    this.Model = 'Atlanta';
}

Here Car.prototype points to an instance of Object because Car is a function and when function declarations are evaluated their prototype is set to an instance of Object.

Car.prototype.Year = 2014; //all Car *instances* will have a Year property set to 2014 on their prototype chain.

var c = new Car(); //create an instance
console.log(c.Year); //2014

Overriding a method present on the prototype chain for an object is as simple as creating a corresponding method on the object:

var myObject = new Object();
myObject.toLocaleString = function() {
  //my own implementation
};

maerics · Accepted Answer · 2014-10-01 20:16:06Z

6

You probably want to modify the constructor function prototype:

function Car(year, make, model) {
  this.year  = year;
  this.make  = make;
  this.model = model;
}

Car.prototype.toLocaleString = function() {
  return [this.year, this.make, this.model].join(' ');
};

var civic = new Car(2014, 'Honda', 'Civic');
civic.toLocaleString(); // => "2014 Honda Civic"

This MDN article on Object.prototype might be helpful.

answered Oct 1, 2014 at 20:16

maerics

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