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Javascript converting "null" to string

Posted on 2009-05-07
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Last Modified: 2012-05-06
I'm working on a Javascript program and I'm trying to figure out the best way to do the following:
I have a string and I want to concatenate it with another variable. The problem is that if the other variable is null, what I wind up with is my first string concatenated with the word "null". Instead, I want to have my first string concatenated with an empty string. Is there any way to change the default Javascript behavior, maybe by changing the toString function or something in String.prototype, so that "casting" a null to a String results in an empty string? Thanks.
var o = { a: 'string', b: null }
document.writeln(o.a + o.b);                     // prints "stringnull", i want it to just print "string"

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Question by:tjgquicken
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8 Comments
 
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Expert Comment

by:secondeff
ID: 24326913
Hi,
What happens if you try using '' instead of null? {a: 'string' , b: ''}
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LVL 12

Expert Comment

by:alien109
ID: 24326984
document.writeln(o.a + ((o.b)?o.b:""));
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LVL 12

Expert Comment

by:Gibu George
ID: 24326994
var o = { a: 'string', b: null };
document.write((o.a==null)?'':o.a+(o.b==null)?'':o.b)
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LVL 10

Accepted Solution

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bugada earned 125 total points
ID: 24327084
var o = { a: 'a', b: null };
document.write((o.a || '') + (o.b || ''));
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Author Comment

by:tjgquicken
ID: 24327686
Bugada probably has the most efficient solution, but I guess that having to check every variable for being null before I use it is just something I'm going to have to get used to in Javascript.
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Author Comment

by:tjgquicken
ID: 24327744
Still I noticed the following:

new String() initializes an empty string
new String(null) initializes the character string "null"
new String(undefined) initializes the character string "undefined"
delete c; new String(c); throws a ReferenceError because c is not defined

There doesn't seem like a lot of logic to this behavior.
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Expert Comment

by:bugada
ID: 24327837
No the behavior is coherent to javascript specifications.

null and undefined are reserved words with a special meaning so if you pass it in the string constructor  it creates an object with these values. This strings print their literal value as output, but you can't do normal operation on them on them, for example a substring.

The default String constructor creates an empty string.

The last line is correct because every variable used must be declared in its context, although most browsers initialize undeclared variable as undefined under the window contex. IE is very strict on this issue.
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Author Comment

by:tjgquicken
ID: 24327876
Ok, that makes a lot more sense.
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