Error.prototype.stack, other browsers

**DHTML Kitchen** · May 1, 2007 10:06

Does anyone know of a hacked up JS patch for Error.prototype.stack or a getStack functionality for IE and Safari?

Does Opera have Error.prototype.stack?

Maybe something like:

Code:

Error.getStack = function getStack( error ) {
  if( error.stack ) return error.stack;
 
 // look up the call chain...
var fakeStack = [ ];
for( var caller = arguments.callee.caller; 
        caller != null; caller = callee.caller ) {
   fakeStack.push( caller );
}
return fakeStack;
};

This is just typed in. I need tested library code that I can rely on.

Thank you,

Garrett

**MikeFoster** · May 1, 2007 15:46

Trying the following on Opera9.10 and FF2 on WinXP:

Code:

window.onload = function()
{
  test();
};
function test()
{
  alert(Error.stack + ', ' + Error.prototype.stack + '\n' + arguments.callee.caller);
}

Opera gives:

"undefined, undefined
undefined"

FF gives:

"undefined, undefined
function() {
test();
}"

Not what I expected.

**DHTML Kitchen** · May 2, 2007 01:41

Ok, so "stack" in Error.prototype is false. That should not be false, I think. Just like we can test for "concat" in Array.prototype;

But we can still inspect an error instance to see if it has a stack property.

you can test this using a try/catch.

Code:

Error.getStack = function getStack( error ) {
  if( error.stack ) return error.stack; // native support.

If opera doesn't support arguments.callee, how can we get a reference to the function of context?

Does Opera support arguments.caller? How about Function.prototype.caller?

Code:

// this was ecma-deprecated. Does opera support?
var f = someFunction; 
caller = someFunction.caller; 
callerCaller = caller.caller;

**MikeFoster** · May 2, 2007 09:41

My test follows. It is also online.

Code:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Test</title>
<script type='text/javascript'>
window.onload = function()
{
  test1();
  test2();
};
function test1()
{
  try {
    var x = window.object_that_doesnt_exist.value;
  }
  catch (e) {
    msg('Test 1');
    msg('e.stack = ' + e.stack);
    msg('e.prototype = ' + e.prototype);
  }
}
function test2()
{
  msg('Test 2');
  msg('arguments.caller = ' + arguments.caller);
  msg('test2.caller = ' + test2.caller);
  msg('arguments.callee = ' + arguments.callee);
  msg('test2.callee = ' + test2.callee);
}
function msg(s)
{
  document.getElementById('msg').innerHTML += '<p>' + s + '<\/p>';
}
</script>
</head>
<body>
<h1>Test</h1>
<h2>Results for Opera 9.10 on WinXP</h2>
<pre>
Test 1

e.stack = undefined

e.prototype = undefined

Test 2

arguments.caller = undefined

test2.caller = undefined

arguments.callee = function test2() { msg("Test 2"); msg("arguments.caller = " + arguments.caller); msg("test2.caller = " + test2.caller); msg("arguments.callee = " + arguments.callee); msg("test2.callee = " + test2.callee); } 

test2.callee = undefined
</pre>
<h2>Results for FF 2.0 on WinXP</h2>
<pre>
Test 1

e.stack = test1()@file://localhost/C:/Documents&#37;20and%20Settings/mfoster_local/My%20Documents/www/Support/test2.html:16 ([object Event])@file://localhost/C:/Documents%20and%20Settings/mfoster_local/My%20Documents/www/Support/test2.html:10 @:0

e.prototype = undefined

Test 2

arguments.caller = undefined

test2.caller = function () { test1(); test2(); }

arguments.callee = function test2() { msg("Test 2"); msg("arguments.caller = " + arguments.caller); msg("test2.caller = " + test2.caller); msg("arguments.callee = " + arguments.callee); msg("test2.callee = " + test2.callee); }

test2.callee = undefined
</pre>
<h2>Results for IE 7 on WinXP</h2>
<pre>
Test 1

e.stack = undefined

e.prototype = undefined

Test 2

arguments.caller = [object Object]

test2.caller = function() { test1(); test2(); }

arguments.callee = function test2() { msg('Test 2'); msg('arguments.caller = ' + arguments.caller); msg('test2.caller = ' + test2.caller); msg('arguments.callee = ' + arguments.callee); msg('test2.callee = ' + test2.callee); }

test2.callee = undefined
</pre>
<h2>Results for IE 6 on WinNT</h2>
<pre>
Test 1

e.stack = undefined

e.prototype = undefined

Test 2

arguments.caller = [object Object]

test2.caller = function() { test1(); test2(); }

arguments.callee = function test2() { msg('Test 2'); msg('arguments.caller = ' + arguments.caller); msg('test2.caller = ' + test2.caller); msg('arguments.callee = ' + arguments.callee); msg('test2.callee = ' + test2.callee); }

test2.callee = undefined
</pre>
<h2>Test Messages</h2>
<div id='msg'></div>
</body>
</html>

I forgot to add "test2.prototype.caller" to the test, but it is undefined in Opera.

**DHTML Kitchen** · May 2, 2007 12:21

added test2.caller to the test code.

Code:

function test2()
{
  msg('Test 2');
  msg('arguments.caller = ' + arguments.caller);
  msg('test2.caller = ' + test2.caller);
  msg('arguments.callee = ' + arguments.callee);
  msg('test2.callee = ' + test2.callee);
  msg('test2.caller = ' + test2.caller);
}

Looks like Opera9 supports neither Function.prototype.caller nor arguments.caller. Although arguments.caller was deprecated, Function.prototype.caller was not.

I don't know of any another way to reference the caller.

Safari: Result is the same as Opera.

Code:

Test 2

arguments.caller = undefined

test2.caller = undefined

arguments.callee = function test2() { msg("Test 2"); msg("arguments.caller = "+arguments.caller); msg("test2.caller = "+test2.caller); msg("arguments.callee = "+arguments.callee); msg("test2.callee = "+test2.callee); msg("test2.caller = "+test2.caller); }

test2.callee = undefined

test2.caller = undefined

**DHTML Kitchen** · May 2, 2007 12:23

It should be possible to get a stack trace from IE, at least.

**DHTML Kitchen** · May 2, 2007 12:43

Filed a bug for Opera. Function.prototype.caller should return function's caller.

**mrhoo** · May 2, 2007 13:26

caller has been deprecated in ecmaScript for years- implementations are encouraged NOT to use it.

I always check for it in a new browser version. So far, IE and firefox still support it, and Opera doesn't. I don't know about Konquerer and Safari-

**DHTML Kitchen** · May 2, 2007 16:32

arguments.caller was deprecated.

Function.prototype.caller is a non-standard feature, but is essential for practical debugging.

http://developer.mozilla.org/en/docs...unction:caller
https://bugzilla.mozilla.org/show_bug.cgi?id=65683

**DHTML Kitchen** · May 2, 2007 16:35

This feature is also essential for enforcing entryPoints, or "access modifiers" as their called in java. private, protected.

It is necessary for lazy-load singleton.

**MikeFoster** · May 2, 2007 17:50

Interesting stuff!

Thanks mrhoo and Garrett!

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