Perfection Kills

Object; // (Internal Function)
document.getElementById; // [function]
(function foo(){ return a+b; }); // function(){ return a+b; }

window.focus; // function () { [native code] }

window.XMLHttpRequest;
// function XMLHttpRequest() { [native code for XMLHttpRequest.XMLHttpRequest, arity=1] }

function foo(){ /* ~10 lines of code */ };
// function foo() { \\Source code is not available. [1035593284ED8CD9D0734E9B14EF4F3FF6BE9686] }

Array.prototype.push; // function () { [native code] }

document.getElementById; // function getElementById() { /* source code not available */ }

function f(){}; // function f() { [cajoled code] }
var f = function(){}; // function f$_var() { [cajoled code] }

( /* foo bar */ (((function f(){}) /* baz qux */ ))); // ( /* foo bar */ (((function f(){}) /* baz qux */ )))

({}).hasOwnProperty; // function hasOwnProperty() { [native code for Object.hasOwnProperty, arity=1] }

  String(function f(x, y) x + y); // function f(x, y) x + y

(function(){ return x + y; }).bind(); // "function () { "[native code]" }"

Comments (5)

1. 

   DBJ said:

   #

   This might be on the edge of this theme. But it concerns one function that is “there” but you can not reach it.
   Explanation. IE8 javascript does this is OK and as ES3 specifies.
   
o = {}
typeof o.f === undefined
Object.prototype.toString.call(o.f) === "[object Object]"

   But … IE has ActiveX controls. We would love to ignore them. But, there is one which we have to use.
   XMLHttpRequest … The problem is very subtle, and as a such it can generate very subtle bugs.
   The issue is that in IE (vs others) you can not call the XMLHttpRequest. In IE it is of a type “object”.
   
// the source of trouble
typeof XMLHttpRequest === "object" // only in IE ?
// it can not be called
XMLHttpRequest()
/*
TypeError
Number : 5
Description : Invalid procedure call or argument
*/
// it must be new-ed, because it is an ActiveX (COM) object
new XMLHttpRequest()
// OK

   And (lo and behold) this is a source of a lot of trouble and workarounds.
   Especially in libraries and any trying to be generic javascript.
   
typeof XMLHttpRequest.toString
/* unknown */
Object.prototype.toString.call(XMLHttpRequest.toString)
/*
TypeError
Number : 438
Description : Object doesn't support this property or method
*/


   IE team forgot to handle “unknown” in Object.prototype.toString.call() ?
   But. Why having “unknown” at all? Why not treat it as undefined, and no confusion ?

   
// how IE+javascript should behave to be more "ES3"
typeof XMLHttpRequest.toString === undefined
Object.prototype.toString.call(XMLHttpRequest.toString) == "[object Object]"

   Even better why not just making it a native global function ? As everybody else, as far as I know?
   
typeof window.XMLHttpRequest === "function"

   Bad decision was made here, I am affraid : http://blogs.msdn.com/ie/archive/2006/01/23/516393.aspx

2. 

   eisd said:

   #

   DBJ:

   The “unknown” type is not restricted to only the XHR object, and it is not a bug on the JScript team’s part that Object.prototype.toString.call does not handle “unknown.” The specification clearly allows for host objects, and the XHR object and other ActiveXObjects should be classified as such, and we should therefore expect the behavior to be erratic and to not necessarily follow the specification.

   When the typeof operator returns “unknown” for an ActiveXObject, it means the property or method exists, but JScript does not know exactly what type it is; whereas if typeof returns “undefined,” it literally means there’s nothing there. However, typeof on a non-existent ActiveXObject property will indeed return “undefined” as per the specification.

   Lastly, Object.prototype.toString.call(ArbitraryActiveX.property) won’t behave the way you expect it to behave (as the specification warns about host objects). Properties and methods of ActiveXObjects are not the same as native Javascript object properties and methods; hence why you are experiencing the aforementioned errors.

Trackbacks

1. links for 2009-06-03 | burningCat said:

   [...] Those tricky functions! [...]

2. JScript and DOM changes in IE9 preview 3 - 隐遁峰 said:

   [...] Note how function representation does not include identifier (f), parameters (x and y), nor representation of function body (return this;). This of course proves once again that relying on function decompilation is NOT a good idea. [...]

3. Perfection kills » JScript and DOM changes in IE9 preview 3 said:

   [...] Note how function representation does not include identifier (f), parameters (x and y), nor representation of function body (return this;). This of course proves once again that relying on function decompilation is NOT a good idea. [...]

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