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Another one from the "this is not a syntax error" department.

The front-end developer is a strange beast who has to jiggle to and fro and code in several languages literally at the same time - javascript, html, css, php or some other server side language, some SQL dialect… No wonder that sometimes we make silly mistakes like:

var $myarray;
var array = array();
$myarray = [];
foreach(var i in myarray)

Last night I just did a silly mistake like this. In JavaScript I used the PHP way of concatenating strings. Something like:

var user = 'Stoyan';
alert('hello ' . user);

This is obviously wrong, but the thing is that it's not a syntax error as one might expect. It alerts "undefined". Why is that?

Well, 'hello' is a string object. You can call methods and properties on it, like:

>>> 'hello'.toUpperCase()
"HELLO"
>>> 'hello'.length
5

And spaces don't matter…

>>> 'hello'     .   length
5
>>> 'hello'  . toUpperCase()
"HELLO"

So 'hello' . user is an attempt to access the "user" property of the string object 'hello'. This property doesn't exist, hence the "undefined" result.

Doing the opposite (using JavaScript-type concatenation in PHP) is also not an error:

$user = 'Stoyan';
echo 'Hello ' + $user; // prints 0

This entry was posted on Thursday, October 25th, 2007 and is filed under JavaScript, php. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.