There was a question on here already about overriding toString and the answer is this:

function Foo() {}

Foo.prototype.toString = function()
{
    return "this is Foo";
}

However my question is:

Is there a way to put this prototype override INSIDE of the object declaration like so:

function Foo() {
    Foo.prototype.toString = function(){
        return "this is Foo";
    }
}

Or like so:

var Foo =  {
Foo.prototype.toString = function(){
    return "this is Foo";
   }
}

It's really weird/annoying to have to put the prototype outside of the object declaration, but maybe this is a quirk of JavaScript. I have been wondering this for awhile.

There was a question on here already about overriding toString and the answer is this:

function Foo() {}

Foo.prototype.toString = function()
{
    return "this is Foo";
}

However my question is:

Is there a way to put this prototype override INSIDE of the object declaration like so:

function Foo() {
    Foo.prototype.toString = function(){
        return "this is Foo";
    }
}

Or like so:

var Foo =  {
Foo.prototype.toString = function(){
    return "this is Foo";
   }
}

It's really weird/annoying to have to put the prototype outside of the object declaration, but maybe this is a quirk of JavaScript. I have been wondering this for awhile.

• javascript
• node.js

Share Improve this question Follow edited Dec 23, 2014 at 17:37 JasonMArcher 15k2222 gold badges5959 silver badges5353 bronze badges asked Dec 23, 2014 at 4:07 Alexander MillsAlexander Mills 100k166166 gold badges536536 silver badges916916 bronze badges

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5 Answers 5

Sorted by: Reset to default 10

As Ted Hopp mentions, there is no other way to do this yet. In ECMAScript 6 you will be able to use the class syntax, which is just syntactict sugar for constuctor functions and prototypes:

class Foo {
  toString() { ... }
}

For object literals, there will also be a shorthand:

var foo = {
  __proto__: {
    toString() {...}
  }
};

The special __proto__ declaration allows you to define the prototype inside the object literal. However, since the protoype is only really useful when it is shared by multiple onjects, directly assigning the property to the object instead would be simpler in this case, and also works today already:

var foo = {
  toString: function() {},
};

I think you are searching for bellow

function Foo() {
    this.toString = function() {
        return "this is Foo";
    }
}

Note:- A drawback of this way is that the method toString() is recreated every time you create a new instance of Foo.

There's no good way to do what you want. The Function object Foo (which is usually an object constructor) doesn't exist until the declaration is evaluated, so there's no prototype available for assigning a toString property. Furthermore, the toString assignment won't be evaluated until the constructor is executed at least once.

In any case, you don't want to be initializing the toString property of the prototype inside the constructor because that will assign a new function for each instance of Foo that is created. You could test whether the prototype already has its own toString property already, but this makes the cure worse than the disease. Just stick to the recipe.

There is no way to do that because at the time you are referencing Foo inside the object, Foo hasn't been declared yet. You could try doing this instead.

var Foo = {}; //assign Foo to an empty object.

and then add various properties/methods to Foo using the Object.defineProperties and Object.assign depending on what you want.

function Foo() {
    return {
        toString: function () {
            return "this is foo";
        }
    };
}

var foo = new Foo();
console.log(foo.toString());