javascript prototype inheritance overwriting methods

Question

Here's what I have now

var Proto = function(){
    this.up = function(){ alert('hello'); };
};

Proto.prototype.up = function(){ alert('world'); };

new o = Proto();

alert(o.up); // prints "hello"

I would expect this to print "world" since I overwrite the method. Any help is appreciated.

Answer 1

See the following snippet:

var MyClass = function () {
    this.print = function logAttachedToThis() { console.log('hello'); };
};
MyClass.prototype.print = function logAttachedToPrototype() { console.log('world'); };
console.log(new MyClass());

Output will be:

MyClass {print: ƒ}
print: ƒ printAttachedToThis()
__proto__:
  print: ƒ printAttachedToPrototype()
  constructor: ƒ ()
  __proto__: Object

When invoking new MyClass().print(), the engine will first check whether print is available in the object itself. Otherwise, it will check in the prototype chain. So actually, this.print = ... is overriding MyClass.prototype.print = .... Not sure overriding is the right word in this specific case though, I would use hiding instead.