Javascript redefine and override existing function body

Question

I am wondering can we still change the function body once it is constructed ?

     var O = function(someValue){
           this.hello = function(){
                 


        

Answer 1

This is because when you access a property of an Object, JavaScript check the properties of the objects first, before going into its prototype.

This is analogous to Java's derived class overriding the base class functionality.

For a better understanding check the code example in Inheriting properties

Also note that someValue in your case is local to the constructor function. If you need it in other functions, you should assign it to this.someValue inside the constructor.

You'll be able to override the hello function for a particular Object here like the following. But not for the entire Class.

i.hello = function(){ console.log('even here someValue is not accessible');};

  var O = function(someValue){
       this.someValue = someValue;
       this.hello = function(){
            return "hello, " + someValue;
       }
 }

 var i = new O("chris");
 console.log(i.hello()); // prints hello, chris
 i.hello = function() { 
   return 'Hi there '+ this.someValue;
 }
 console.log(i.hello()); // prints Hi there chris

 var test = new O('Sujay')
 console.log(test.hello()) // this still prints hello, Sujay

Note that here we have not changed the constructor, and hence this will not work with other instances like test in the above example.

The best way to do it would be to define functions only in the prototype & not in the constructor, like the following snippet.

 var O = function(someValue){
      this.someValue = someValue;
 };
 O.prototype.hello = function(){
            return "hello, " + this.someValue;
 };

 var i = new O("chris");
 console.log(i.hello()); // prints hello, chris
 O.prototype.hello = function() { 
   return 'Hi there '+ this.someValue;
 }
 console.log(i.hello()); // prints Hi there chris

 var test = new O('Sujay')
 console.log(test.hello()) // prints Hi there Sujay