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Message-ID: <1934328.aqSPkdTlGX@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Mon, 23 Jan 2012 13:37:45 +0100
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Subject: Re: Type undefined
Newsgroups: comp.lang.javascript
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Jake Jarvis wrote:

> On 23.01.2012 11:23, wrote Archos:
>> Why I get an error about a type undefined when it has been defined in
>> top of function Older10() ?
>> 
>> ===
>> function person(name, age) {this.name=name; this.age=age;}
>> 
>> function Older10(people) {
>>   var old = people[0];
>> 
>>   for (var index = 1; index < 10; index++) {
>>     if (people[index] > old.age) {
>>       old = people[index]; // ERROR! TypeError: old is undefined
>>     }
>>   }
>>   return old;
>> }
>> 
>> function main() {
>> var array = new Array(10);
>> 
>> array[1] = new person("Paul", 23);
>> array[2] = new person("Jim", 24);
>> array[3] = new person("Sam", 84);
>> array[4] = new person("Rob", 54);
>> array[8] = new person("Karl", 19);
>> 
>> var older = Older10(array);
>> 
>> console.log("The older of the group is: " + older.name + "\n");
>> }
>> ===
> 
> 'array[0]' in function 'main' is undefined.
> Therefore 'people[0]' in function 'Older10' is undefined.

Correct.

However, the TypeError exception is _not_ thrown at the line that the OP 
marked, but one line *before*, when with `old.age' it is attempted to access 
the property named `age' of the value `undefined'.  Whereas `undefined' is a 
primitive value (the sole value of the Undefined type) that cannot be 
wrapped into a temporary object [1], and therefore has no properties [2] 
(the same happens with `null', the sole value of the Null type [1, 3]).

If that property access had not occurred, the value of `old' would have been 
overwritten with the value of `people[index]', and no exception would have 
been thrown (implementations of ECMAScript Editions 1 to 3, and 5.x use 
*loose typing* [4]).


PointedEars
___________
[1] ECMAScript Language Specification, Edition 5.1 (ES 5.1), section 11.2.1,
    step 5, and therefore section 9.10
[2] ES 5.1, section 4.3.10
[3] ES 5.1, section 4.3.12
[4] ES 5.1, section 9
-- 
> If you get a bunch of authors […] that state the same "best practices"
> in any programming language, then you can bet who is wrong or right...
Not with javascript. Nonsense propagates like wildfire in this field.
  -- Richard Cornford, comp.lang.javascript, 2011-11-14