Error-free deep property access?
Matt Kruse
access that will not throw an error if any property along the chain is
undefined?
For example:
deep(document, "body.firstChild.style.backgroundColor");
or
deep("myElementId.childNodes[3].id");
or
deep( myJsonObject,
"locations.ca.sandiego.directory.people.smith.john.phone");
This would return undefined if any element along the chain is not
there, or index is out of bounds, etc. If the chain gets to the end,
it would return the last-evaluated value. This would be convenient for
accessing deep properties of objects that may or may not exist, and
not having to manually check the chain in your code.
I'm curious to know if anyone uses such an approach, or the cons of
doing so.
Matt Kruse
Scott Sauyet
> Does anyone here use a general convenience method for deep property
> access that will not throw an error if any property along the chain is
>
> I'm curious to know if anyone uses such an approach, or the cons of
> doing so.
I used such a technique only once. It was easy enough to write,
performed well enough for my uses, and did simplify some code. But as
new developers came aboard the project, it was one more thing that
they hadn't seen before, and it rarely turned out to be useful enough
to justify even the short learning curve.
But then again I don't think I've ever written something as deep as
your third example.
-- Scott
Thomas 'PointedEars' Lahn
> Does anyone here use a general convenience method for deep property
> access that will not throw an error if any property along the chain is
> undefined?
>
> For example:
>
> deep(document, "body.firstChild.style.backgroundColor");
> or
> deep("myElementId.childNodes[3].id");
> or
> deep( myJsonObject,
> "locations.ca.sandiego.directory.people.smith.john.phone");
getFeature() and isMethod() in JSX:object.js do a similar thing. They could
be rewritten to support your requirements using dotsToBrackets() in
JSX:types.js. Note that I had opted for several arguments instead because
property names may contain `.', `[', or `]'.
PointedEars
--
Danny Goodman's books are out of date and teach practices that are
positively harmful for cross-browser scripting.
-- Richard Cornford, cljs, <cife6q$253$1$8300...@news.demon.co.uk> (2004)
Matt Kruse
> Does anyone here use a general convenience method for deep property
> access that will not throw an error if any property along the chain is
> undefined?
Well, here's my stab at it:
/*
Error-Free Deep Property Access!
Returns: Property value, or undefined if any part of the property
chain is undefined
Usage:
$prop( object, 'property_name' )
$prop( 'element_id.property_name' )
$prop( 'element_id.prop1.prop2.prop3' )
$prop( object, 'array_property[0]' )
$prop( object, 'method(arg).property_name.array_property[0]
[1].prop' )
$prop( window, 'document.getElementsByTagName(div)
[0].childNodes[1].style.color' )
Special Usage:
$prop() returns the object last evaluated!
if ($prop("id.style.color")) {
alert( $prop() );
}
JSON Example:
var json = {
'a':'1'
,'b': ['x','y','z']
,'c': {
'array':['1','2','3']
,'property':'prop!'
}
}
$prop(json,'a') ==> 1
$prop(json,'b[1]') ==> y
$prop(json,'c.array[2]') ==> 3
$prop(json,'d.e.f.g') ==> undefined
*/
var $prop = (function() {
var last_match;
return function(a,b) {
// Calls to $property() return the last match
if (typeof a=="undefined") { return last_match; }
var context, props = null, p;
// If first arg is not a string, assume it's an object to
// start from
if (typeof a!="string") {
context = a;
props = b.split(".");
}
// Otherwise it's just a string where the first part is an
// element ID
else {
props = a.split(".");
context = document.getElementById(props.shift());
if (!context) { return; }
}
while (context && props.length>0) {
if (context==null) { return; }
p = props.shift();
// If there is an array index [i] at the end, only
// process the first part and stick the second part
// back on the beginning
if ( p.match(/(.+?)(\[\d+\].*)/) ) {
p = RegExp.$1;
props.unshift(RegExp.$2);
}
// if the first part itself is an array index [i]
// then process it
if ( p.match(/^\[(\d+)\]$/) ) {
if (!context || !context.length) { return; }
context = context[RegExp.$1];
}
// If it's a function(arg) call
else if ( p.match(/(.*)\((.*?)\)/) ) {
context = context[RegExp.$1](RegExp.$2);
}
// Else it's a regular property
else {
context = context[p];
}
if (typeof context=="undefined") { return; }
}
last_match = context;
return context;
}
})();
Matt Kruse
Scott Sauyet
> On May 11, 8:44 am, Matt Kruse <m...@thekrusefamily.com> wrote:
>
>> Does anyone here use a general convenience method for deep property
>> access that will not throw an error if any property along the chain is
>> undefined?
>
So this won't allow, for instance,
$props("myObj.prop1.prop2[current.value]")
Is that right? It needs to have only a single root at the first
token. That will cover a lot of possibilities, but the more general
case is interesting too.
--
Scott
Matt Kruse
> So this won't allow, for instance,
> $props("myObj.prop1.prop2[current.value]")
> Is that right? It needs to have only a single root at the first
> token. That will cover a lot of possibilities, but the more general
> case is interesting too.
True. But since you're passing in a string, couldn't you just do:
$prop("myObj.prop1.prop2."+current.value)
?
Matt
Thomas 'PointedEars' Lahn
Generally, no. Think about it. See also the caveats that I have mentioned.
PointedEars
--
Anyone who slaps a 'this page is best viewed with Browser X' label on
a Web page appears to be yearning for the bad old days, before the Web,
when you had very little chance of reading a document written on another
computer, another word processor, or another network. -- Tim Berners-Lee
Scott Sauyet
Sure, unless current is undefined, which I think is the original
problem we're trying to solve. :-)
And of course we could want
$props("myObj.prop1.prop2[current.deeply.nested.value]")
as well.
--
Scott
Thomas 'PointedEars' Lahn
> Matt Kruse wrote:
>> On May 11, 12:19 pm, Scott Sauyet <scott.sau...@gmail.com> wrote:
>>> So this won't allow, for instance,
>>> $props("myObj.prop1.prop2[current.value]")
>>> Is that right? It needs to have only a single root at the first
>>> token. That will cover a lot of possibilities, but the more general
>>> case is interesting too.
>>
>> True. But since you're passing in a string, couldn't you just do:
>>
>> $prop("myObj.prop1.prop2."+current.value)
>> ?
>
> Sure, unless current is undefined, [...]
Wrong. Think about what would happen with
current.value = 42;
or
current.value = "foo.bar";
or
current.value = "foo['bar']";
Thomas 'PointedEars' Lahn
> if ( p.match(/(.+?)(\[\d+\].*)/) ) {
> p = RegExp.$1;
> props.unshift(RegExp.$2);
Other flaws of this approach that I have already mentioned aside, the `$n'
properties of `RegExp' are deprecated as of JavaScript 1.5 at least. Use
the return values of String.prototype.match() and RegExp.prototype.exec(),
respectively, instead.
<https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Deprecated_Features#RegExp_Properties>
Asen Bozhilov
> Does anyone here use a general convenience method for deep property
> access that will not throw an error if any property along the chain is
> undefined?
>
> For example:
>
> deep(document, "body.firstChild.style.backgroundColor");
> or
> deep("myElementId.childNodes[3].id");
Square bracket notation allow to used returned result by expression,
and that result is not bound as Identifiers rules defined by ECMA-262.
With your strategy you would have problems. For example:
var obj = {};
obj['.prop.'] = true;
obj['[prop]'] = true;
print(obj['.prop.']);
print(obj['[prop]'])
Your strategy is too complex and implementation will be terrible any
time. For example you can use evil for your purposes:
eval(properties);
Of course that is not solve the design problems. I will prefer to use
array which contain each property name. For example:
deep(context, ['property1', 'property2', 'propertyN']);
Matt Kruse
> > True. But since you're passing in a string, couldn't you just do:
> > $prop("myObj.prop1.prop2."+current.value)
> > ?
> Sure, unless current is undefined, which I think is the original
> problem we're trying to solve. :-)
I think it's good to solve the most general case that is reasonable
and useful, but not EVERY general case.
> And of course we could want
> $props("myObj.prop1.prop2[current.deeply.nested.value]")
> as well.
Sure, you COULD do that, but I don't imagine I ever WOULD. So solving
that case holds little value for me. But using my function, I suppose
you could do:
if ( $prop("current.deeply.nested.value") ) {
$prop("myObj.prop1.prop2."+$prop())
}
Matt Kruse
Matt Kruse
> Square bracket notation allow to used returned result by expression,
> and that result is not bound as Identifiers rules defined by ECMA-262.
> With your strategy you would have problems. For example:
> var obj = {};
> obj['.prop.'] = true;
> obj['[prop]'] = true;
>
> print(obj['.prop.']);
> print(obj['[prop]'])
>
> Your strategy is too complex
But why would you EVER do that? I'm not building this for idiots!
> and implementation will be terrible any
> time.
I don't know, my implementation is pretty straight-forward and works
well.
> For example you can use evil for your purposes:
> eval(properties);
Which will still throw errors.
> I will prefer to use
> array which contain each property name. For example:
> deep(context, ['property1', 'property2', 'propertyN']);
How is that any better than joining the array with "." and passing a
single string?!
Matt Kruse
Garrett Smith
> Matt Kruse wrote:
>
[...]
> eval(properties);
>
> Of course that is not solve the design problems. I will prefer to use
> array which contain each property name. For example:
>
> deep(context, ['property1', 'property2', 'propertyN']);
>
Does deep check own or prototype?
--
Garrett
comp.lang.javascript FAQ: http://jibbering.com/faq/
Thomas 'PointedEars' Lahn
> Asen Bozhilov wrote:
>> Square bracket notation allow to used returned result by expression,
>> and that result is not bound as Identifiers rules defined by ECMA-262.
>> With your strategy you would have problems. For example:
>> var obj = {};
>> obj['.prop.'] = true;
>> obj['[prop]'] = true;
>>
>> print(obj['.prop.']);
>> print(obj['[prop]'])
>>
>> Your strategy is too complex
>
> But why would you EVER do that?
It can be necessary for mapping values that property names contain dots or
brackets.
> I'm not building this for idiots!
ISTM you are not building this for people who do DOM scripting either.
<form>...<input name="foo[]">...</form>
> [...]
>> I will prefer to use array which contain each property name. For example:
>> deep(context, ['property1', 'property2', 'propertyN']);
>
> How is that any better than joining the array with "." and passing a
> single string?!
The property names are clear.
PointedEars
--
var bugRiddenCrashPronePieceOfJunk = (
navigator.userAgent.indexOf('MSIE 5') != -1
&& navigator.userAgent.indexOf('Mac') != -1
) // Plone, register_function.js:16
Thomas 'PointedEars' Lahn
> Asen Bozhilov wrote:
>> eval(properties);
>>
>> Of course that is not solve the design problems. I will prefer to use
>> array which contain each property name. For example:
>>
>> deep(context, ['property1', 'property2', 'propertyN']);
>
> Does deep check own or prototype?
You want to re-read the thread and re-think your question.
Scott Sauyet
It's more general, not necessarily better. But it does address the
issues Thomas raises, and allows for the generality I was suggesting.
If your simple cases are all you ever need, by all means use a simpler
strategy.
It also has the advantage that you can pass variables holding strings
without manual concatenation. The only thing I would do differently
than the above is that I would remove the array and deal with variable
arguments; this seems cleaner to me:
deep(context, 'property1', 'property2', 'propertyN');
This still does not get at the even more general solution I mentioned,
and which I tried to implement some time ago. But of course that
solution had the problem yours does with property names containing
periods or brackets. I used an escape syntax for those, and that's
where some of the clean API went up in smoke.
I will see if I still have a copy of that code at home. I'm curious
to see how much five-year-old code might make me shudder.
--
Scott
Asen Bozhilov
> Asen Bozhilov wrote:
> > I will prefer to use
> > array which contain each property name. For example:
> > deep(context, ['property1', 'property2', 'propertyN']);
>
> How is that any better than joining the array with "." and passing a
> single string?!
At all you miss the point and wrote in arrogant way that reply. Could
you show an implementation which works with:
var obj = {
property : {
'.property.' : {
'[property]' : true
}
}
};
$prop(obj, 'property[.property.][[property]]');
I expect `true' instead of `undefined'. So when you show that
implementation we can again talk about the complex of the problem
which you try to solve.
Matt Kruse
> this seems cleaner to me:
> deep(context, 'property1', 'property2', 'propertyN');
Would it handle this:
deep(context, 'prop1[0]')
if 'context' has no property named 'prop1'?
And what if 'context' has a property named 'myarray[0]' which is an
array?
deep(context, 'myarray[0][0]')
?
Matt Kruse
Garrett Smith
> On May 11, 2:45 pm, Asen Bozhilov <asen.bozhi...@gmail.com> wrote:
>> Square bracket notation allow to used returned result by expression,
>> and that result is not bound as Identifiers rules defined by ECMA-262.
>> With your strategy you would have problems. For example:
>> var obj = {};
>> obj['.prop.'] = true;
>> obj['[prop]'] = true;
>>
>> print(obj['.prop.']);
>> print(obj['[prop]'])
>>
>> Your strategy is too complex
>
> But why would you EVER do that? I'm not building this for idiots!
>
Host:
navigator.plugins["Shockwave Flash 2.0"]?
User defined:
myObject[ prop.toString() ] = val;
[...]
Matt Kruse
> At all you miss the point and wrote in arrogant way that reply.
I don't think the reply was at all arrogant, but we may have a
separation in language.
> Could you show an implementation which works with:
> var obj = {
> property : {
> '.property.' : {
> '[property]' : true
> }
> }
>
> };
> $prop(obj, 'property[.property.][[property]]');
> I expect `true' instead of `undefined'. So when you show that
> implementation we can again talk about the complex of the problem
> which you try to solve.
That is a more complex problem, yes. But it's one that _you_ are
proposing to solve, not me ;)
I will gladly limit the potential situations for which my solution
will apply, which for my case will probably cover 99.9% of the cases.
In the rare cases where it doesn't, I'm happier with writing context-
specific code rather than extending my solution to a more general,
obscure case.
Matt Kruse
Garrett Smith
> Garrett Smith wrote:
>
>> Asen Bozhilov wrote:
[...]
> You want to re-read the thread and re-think your question.
I want you to stop taking your personal frustrations out on this NG.
Thomas 'PointedEars' Lahn
> Scott Sauyet wrote:
>> this seems cleaner to me:
>> deep(context, 'property1', 'property2', 'propertyN');
>
> Would it handle this:
>
> deep(context, 'prop1[0]')
>
> if 'context' has no property named 'prop1'?
No, for it would be looking for a property named `prop1[0]'. To look up the
`0' property of the object referred to by the `prop1' property, you would of
course call it so:
deep(context, 'prop1', 0)
> And what if 'context' has a property named 'myarray[0]' which is an
> array?
>
> deep(context, 'myarray[0][0]')
>
> ?
deep(context, 'myarray[0]', 0)
or
deep(context, ['myarray[0]', 0])
Which one you prefer would depend on the purpose. For example, for
JSX:isMethod/areMethods() I used additional arguments for property names
that are part of the same member expression, and a trailing additional array
argument for names of properties that are properties of the same object,
i.e.
jsx.object.areMethods(foo, 'bar', ['baz', 'bla'])
returns `true' iff both `foo.bar.baz' and `foo.bar.bla' refer to supposedly
callable objects that are referred to by properties of an object (in short:
methods).
Thomas 'PointedEars' Lahn
> Thomas 'PointedEars' Lahn wrote:
>> Garrett Smith wrote:
>>> Asen Bozhilov wrote:
> [...]
>> You want to re-read the thread and re-think your question.
>
> I want you to stop taking your personal frustrations out on this NG.
It was merely a hint, stupid, and you managed to miss it.
Garrett Smith
> Garrett Smith wrote:
>
>> Thomas 'PointedEars' Lahn wrote:
>>> Garrett Smith wrote:
>>>> Asen Bozhilov wrote:
>> [...]
>>> You want to re-read the thread and re-think your question.
>> I want you to stop taking your personal frustrations out on this NG.
>
> It was merely a hint, stupid, and you managed to miss it.
>
Thomas 'PointedEars' Lahn
> Thomas 'PointedEars' Lahn wrote:
>> Garrett Smith wrote:
>>> Thomas 'PointedEars' Lahn wrote:
>>>> Garrett Smith wrote:
>>>>> Asen Bozhilov wrote:
>>> [...]
>>>> You want to re-read the thread and re-think your question.
>>> I want you to stop taking your personal frustrations out on this NG.
>> It was merely a hint, stupid, and you managed to miss it.
>>
> That is exactly what I am talking about.
Calling you stupid when you do stupid things is a telling a fact, not an
expression of any personal frustration I might have. So the problem is on
your part, not on mine.
PointedEars
--
Use any version of Microsoft Frontpage to create your site.
(This won't prevent people from viewing your source, but no one
will want to steal it.)
-- from <http://www.vortex-webdesign.com/help/hidesource.htm> (404-comp.)
Garrett Smith
> Garrett Smith wrote:
>
>> Thomas 'PointedEars' Lahn wrote:
>>> Garrett Smith wrote:
>>>> Thomas 'PointedEars' Lahn wrote:
>>>>> Garrett Smith wrote:
>>>>>> Asen Bozhilov wrote:
>>>> [...]
>>>>> You want to re-read the thread and re-think your question.
>>>> I want you to stop taking your personal frustrations out on this NG.
>>> It was merely a hint, stupid, and you managed to miss it.
>>>
>> That is exactly what I am talking about.
>
> Calling you stupid when you do stupid things is a telling a fact, not an
> expression of any personal frustration I might have. So the problem is on
> your part, not on mine.
>
Asen did not post code; I guess his `deep` does check prototype chain
but it's not something that's been mentioned yet.
Matt Kruse
wrote:
> > I'm not building this for idiots!
> ISTM you are not building this for people who do DOM scripting either.
> <form>...<input name="foo[]">...</form>
This already works fine:
$prop(document, "forms[0].foo[].value")
for example.
Matt Kruse
Matt Kruse
> Could you show an implementation which works with:
> var obj = {
> property : {
> '.property.' : {
> '[property]' : true
> }
> }
>
> };
> $prop(obj, 'property[.property.][[property]]');
> I expect `true' instead of `undefined'. So when you show that
> implementation we can again talk about the complex of the problem
> which you try to solve.
Easy. I just modified the function to accept an optional last
parameter of delimiter.
So in this case you could just do:
$prop(obj, 'property|.property.|[property]', '|')
Maybe not as straight-forward as passing multiple arguments, but same
difference.
Matt Kruse
Thomas 'PointedEars' Lahn
No, by contrast you will not be able to handle property names that contain
`|'.
Thomas 'PointedEars' Lahn
> Thomas 'PointedEars' Lahn wrote:
>> > I'm not building this for idiots!
>> ISTM you are not building this for people who do DOM scripting either.
>> <form>...<input name="foo[]">...</form>
>
> This already works fine:
>
> $prop(document, "forms[0].foo[].value")
>
> for example.
You miss the point.
Matt Kruse
wrote:
> No, by contrast you will not be able to handle property names that contain
> `|'.
Umm, then you just pick a different delimiter that doesn't conflict
with your property names. Duh.
Matt Kruse
Matt Kruse
wrote:
> Matt Kruse wrote:
> > Thomas 'PointedEars' Lahn wrote:
> >> > I'm not building this for idiots!
> >> ISTM you are not building this for people who do DOM scripting either.
> >> <form>...<input name="foo[]">...</form>
> > This already works fine:
> > $prop(document, "forms[0].foo[].value")
> > for example.
> You miss the point.
On the contrary, you fail to make one.
Matt Kruse
David Mark
And you still can't see why you are making a colossal design mistake?
Don't parse strings. Pass multiple arguments or an array of strings.
There you go.
David Mark
No, as usual, you waste endless amounts of time "debating" when you
could be working. How do you ever get anything done?