ie constructor bug.js

/* MODIFY PROTOTYPE */
var _fn=function(){};
_fn.prototype.a=1;
var v=new _fn();

alert(v.a === 1); /* true */
alert(v.constructor === _fn); /* true */


/* REPLACE PROTOTYPE */
var _fn=function(){};
_fn.prototype={a:1};
var v=new _fn();

alert(v.a === 1); /* true */
alert(v.constructor === _fn); /* false in ie; true in other browsers */

/*

This should be `false` in other browsers as well. 
This is because `constructor` property is resolved on `Object.prototype` 
(during `v.constructor` property lookup, constructor is not found on object directly, 
so it's searched in next object in prototype chain, which is `_fn.prototype`; 
it's not found there either, and so search goes on further to `Object.prototype`, 
where `constructor` is finally found; it refers to `Object` there (unless overwritten), 
which is what you're seeing in next line).

*/

/* but */
alert(v.constructor === Object.prototype); /* true in ie. ie set 'constructor' property eq Object if 'prototype' replaced

                                               ^--- It doesn't really set anything. 
                                               `constructor` is found through prototype chain during property lookup.

REPLACE PROTOTYPE WITH FIXING BUG OF IE */
var _fn=function(){};
_fn.prototype={a:1};

/*
                ^--- or you can just add "constructor" here, and reference proper object in it.
                     e.g.: _fn.prototype = { a: 1, constructor: _fn };

*/

var v=new _fn();
v.constructor=_fn; /* set 'constructor' property by hands */

alert(v.a === 1); /* true */
alert(v.constructor === _fn); /* now true in all browsers */