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Posting Shark

Re: 'null' is null or not an object

#11

Aug 1st, 2008

Well, yeah. ( undefined == null ) only works via type coercion.
IMO, more useful ( and pleasant reading ) than either is to coerce straight to boolean;

if( x ) {
// where x may be null, undefined, or an object
}

But only because I'd rarely make a meaningful working distinction between ( or differently handle ) an object variable being null or being undefined.

Plato forgot the nullahedron..

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Lazy, Useless & Apathetic

Re: 'null' is null or not an object

#12

Aug 1st, 2008

> But only because I'd rarely make a meaningful working distinction between ( or differently
> handle ) an object variable being null or being undefined.

null is a valid value, undefined is *nothing*. Undefined has a special significance in the sense that attempting to retrieve value for a non-existent key from a Javascript object returns undefined . Simply put, the distinction is important when:

if(x) {
  // do something
}

isn't acceptable. :-)

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Posting Shark

Re: 'null' is null or not an object

#13

Aug 1st, 2008

Of course, there are sometimes occasions where the distinction is meaningful, but === undefined or === null aren't always the most useful, consider:

 Help with Code Tags 
 javascript Syntax (Toggle Plain Text) 
if( x !== undefined ) {
  x.something( );
  //Possible error, if x happens to be null:
  //""null is null or not an object""
  //or something along those lines
}
if( x !== undefined ) {
  x.something( );
  //Possible error, if x happens to be null:
  //""null is null or not an object""
  //or something along those lines
}

Can, of course use if ( ( x !== undefined ) & ( x !== null ) ) , but it doesn't really gain very much over if( x ) .

Using a typeof test ( AND a non-null test ) is probably the most effective, in most places.

Plato forgot the nullahedron..

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Lazy, Useless & Apathetic

Re: 'null' is null or not an object

#14

Aug 1st, 2008

> Can, of course use if ( ( x !== undefined ) & ( x !== null ) ) , but it doesn't really gain
> very much over if( x ) .

Err..no, it very much does since blank strings, the boolean value false and the integer 0 all evaluate to false. Consider:

var person = {name: "sos", hostile: false, description: ''};

if(person["hostile"]) {
  // Even though a mapping exists for they key 'hostile', 
  // the program flow doesn't enter this block.
}

if(person["description"]) {
  // ditto here.
}

Also it is much better to base your test of undefined on typeof rather than comparison operator since comparison fails for non-existent variables. Consider the following two almost similar snippets. The first one doesn't work, the second one does:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
            "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
    <meta http-equiv="Script-Content-Type" content="text/javascript">
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Test</title>
</head>
<body>
  <script type="text/javascript">
  alert(a === undefined);  
  </script>  
</body>
</html>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
            "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
    <meta http-equiv="Script-Content-Type" content="text/javascript">
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Test</title>
</head>
<body>
  <script type="text/javascript">
  alert(typeof a === "undefined");
  </script>  
</body>
</html>

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Posting Shark

Re: 'null' is null or not an object

#15

Aug 1st, 2008

Ouch. I'd rather prefer to know if something is or isn't an object a string, a boolean, a null, etc. Hence typeof. Only testing ( x !== undefined ) and/or ( x !== null ) isn't atall helpful if I still have no idea what I can subsequently do with x.

The test I'd usually use is ( If I have absolutely no idea what x might be, and I'm hoping for an object for example ):

 Help with Code Tags 
 javascript Syntax (Toggle Plain Text) 
if( ( typeof x == "object " ) && x ) {
  //do whatever ( i.e. poke around in x .looking for the method/property I need ) [ the && x is needed because typeof null is "object" ]
}
if( ( typeof x == "object " ) && x ) {
  //do whatever ( i.e. poke around in x .looking for the method/property I need ) [ the && x is needed because typeof null is "object" ]
}

But, more often than not, I'd have a good idea what x is going to be, and would normally only ever use null as placeholder for things that I'm sure are only going to otherwise be objects, so, any tests can be more simply put ( assuming of course, that I control ALL of the functions involved ):

 Help with Code Tags 
 javascript Syntax (Toggle Plain Text) 
var x = get_x_or_null( );
if( ! x ) {
  x = try_something_else_to_get_x_or_null( );
}
//etc, etc.
var x = get_x_or_null( );
if( ! x ) {
  x = try_something_else_to_get_x_or_null( );
}
//etc, etc.

Otherwise, for the heterogenous collection.. I'd try and setup a convention for each value, or use a "real class" with a constructor/properties/etc.

If I'm expecting an object and I pull out a null ( or an undefined ) its probably, although not always, going to amount to the same recovery, e.g.:

 Help with Code Tags 
 Javascript Syntax (Toggle Plain Text) 
//I'm gonna keep _only_ object pointers to Whatever objects in here.
var xx = Object( );
var a = xx[ "a" ];
if( ! a ) {
  //don't care if a is null or undefined, I need to do something with a Whatever.
  xx[ "a" ] = a = new Whatever( );
}
a.do_something( );
//I'm gonna keep _only_ object pointers to Whatever objects in here.
var xx = Object( );
var a = xx[ "a" ];
if( ! a ) {
  //don't care if a is null or undefined, I need to do something with a Whatever.
  xx[ "a" ] = a = new Whatever( );
}
a.do_something( );

But yeh, that only works if you keep the collection homogenous..

Plato forgot the nullahedron..

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Lazy, Useless & Apathetic

Re: 'null' is null or not an object

#16

Aug 1st, 2008

> But, more often than not, I'd have a good idea what x is going to be, and would normally
> only ever use null as placeholder for things that I'm sure are only going to otherwise be
> objects

Again fails if you end up explicitly constructing Boolean, String and Number objects since they suffer with the same drawbacks as their primitive counterparts.

> But yeh, that only works if you keep the collection homogenous..

...which almost never is the case with real world Javascript object literals. :-)

Anyways, the intent of my posts was not to nitpick at the code snippets which almost always work in normal cases, but to make it clear to the OP that dark corners do exist in the happy go lucky world of Javascript.

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Failing to value the lives of others cheapens your own.

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