Javascript add method to object

Question

Suppose I have a Foo object  How can I extend this object by adding a bar   method and also ensure that future instances of Foo have this method

User · Answer

There are many ways to create re-usable objects like this in JavaScript. Mozilla have a nice introduction here:

https://developer.mozilla.org/en-US/docs/JavaScript/Introduction_to_Object-Oriented_JavaScript

The following will work in your example:

function Foo(){
    this.bar = function (){
        alert("Hello World!");
    }
}

myFoo = new Foo();
myFoo.bar(); // Hello World?????????????????????????????????

User · Answer

you need to add it to Foo s prototype   function Foo     Foo prototype bar   function     var x   new Foo   x bar

User · Answer

You can make bar a function making it a method   Foo bar   function passvariable         As a property it would just be assigned a string  data type or boolean  Foo bar    a place

User · Answer

This all depends on how you re creating Foo  and how you intend to use  bar     First  are you using a constructor-function for your object     var myFoo   new Foo      If so  then you can extend the Foo function s prototype property with  bar  like so   function Foo                Foo prototype bar   function                  var myFoo   new Foo    myFoo bar      In this fashion  each instance of Foo now has access to the SAME instance of  bar  To wit   bar will have FULL access to this  but will have absolutely no access to variables within the constructor function   function Foo      var secret   38  this name    Bob     Foo prototype bar   function      console log secret      Foo prototype otherFunc   function      console log this name       var myFoo   new Foo    myFoo otherFunc        Bob   myFoo bar       error --  secret  is undefined                       or a value of  secret  in a higher global scope   In another way  you could define a function to return any object  not this   with  bar created as a property of that object   function giveMeObj          var private   42          privateBar   function      console log private              public interface                 bar   privateBar                 return public interface     var myObj   giveMeObj    myObj bar       42   In this fashion  you have a function which creates new objects  Each of those objects has a  bar function created for them  Each  bar function has access  through what is called closure  to the  private  variables within the function that returned their particular object  Each  bar still has access to this as well  as this  when you call the function like myObj bar    will always refer to myObj  public interface  in my example Foo      The downside to this format is that if you are going to create millions of these objects  that s also millions of copies of  bar  which will eat into memory   You could also do this inside of a constructor function  setting this bar   function        inside of the constructor -- again  upside would be closure-access to private variables in the constructor and downside would be increased memory requirements   So the first question is  Do you expect your methods to have access to read modify  private  data  which can t be accessed through the object itself  through this or myObj X      and the second question is  Are you making enough of these objects so that memory is going to be a big concern  if you give them each their own personal function  instead of giving them one to share     For example  if you gave every triangle and every texture their own  draw function in a high-end 3D game  that might be overkill  and it would likely affect framerate in such a delicate system       If  however  you re looking to create 5 scrollbars per page  and you want each one to be able to set its position and keep track of if it s being dragged  without letting every other application have access to read set those same things  then there s really no reason to be scared that 5 extra functions are going to kill your app  assuming that it might already be 10 000 lines long  or more

[javascript] Javascript add method to object

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