Peter Chang

@peterchang_82818

[ Javascript ] Cracking nuts, override Object.constructor( )

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I like javascript, as always. But it was hurt, every time when I was required to program in OOP pattern. All those topics: Objects, Classes,Prototype, Inheritance etc., are the pain for developer, which caused by the personality of “loose comparison temper” and “auto type conversion OCD” of JS.

This is an example from stackoverflow:

First, Rabbit is a object, with public valuable jumps and default value “yes”.

function Rabbit() {
this.jumps = "yes";
};
var rabbit = new Rabbit();
console.log(rabbit.jumps); // yes
console.log(Rabbit.prototype.constructor);
// outputs exactly the code of the function Rabbit();

Second, we want to change the code in Rabbit() so that the var jumps with different default value “no”. Well, modifying default value of Object, and there is Object.prototype.constructor( ), my gut tells me modifying the constructor function and problem will solved. BUT it is not.

Rabbit.prototype.constructor = function Rabbit() {
this.jumps = "no";
};
console.log(Rabbit.prototype.constructor);
// again outputs the code with new this.jumps = "no";
var rabbit2 = new Rabbit();
// create new object with new constructor
console.log(rabbit2.jumps); // still
yes

What is happening is that Rabbit.prototype.constructor is just a pointer to the original constructor (function Rabit(){…}), so that users of the ‘class’ can detect the constructor from an instance.

By Juan

If you want to redefine a constructor, just do

var oldProto = Rabbit.prototype;
Rabbit = function() {
this.jumps = "no";
};
Rabbit.prototype = oldProto;

You also like Cracking nuts series:

[Expressjs] override res.send
[Expressjs] put the IP to BlackList

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Reference:

Why is it impossible to change constructor function from prototype?
http://js-bits.blogspot.tw/2010/08/javascript-inheritance-done-right.html

Tuesday, August 17, 2010 Javascript Inheritance Done Right
http://stackoverflow.com/questions/9267157/why-is-it-impossible-to-change-constructor-function-from-prototype

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