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How do return object properties work in JavaScript?

The code

var Foo = function(value) {
  this.val = value;
}

var foo = new Foo(2);
var boo = new Foo(3);
return foo+boo;

returns [object Object][object Object]. It makes sense to me because they are objects. But this modified version of the code returns 5:

var Foo = function(value) {
  this.val = value;
}

Foo.prototype.valueOf = function() {
  return this.val;
}

var foo = new Foo(2);
var boo = new Foo(3);
return foo+boo;

Why? I would think that to obtain such a result would require foo.valand boo.val. I would also think what would valueOfneed to be called so that it could affect the return values.

I looked at the JavaScript documentation pertaining to valueOf()the Mozilla Developer Network (it is recommended to read this problem), but I still don't understand what is going on here. (Side note: I also don't understand how a function can be called without the brackets following it ...)

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