Whether a variable is undefined
JqueryJquery Problem Overview
How do I find if a variable is undefined?
I currently have:
var page_name = $("#pageToEdit :selected").text();
var table_name = $("#pageToEdit :selected").val();
var optionResult = $("#pageToEditOptions :selected").val();
var string = "?z=z";
if ( page_name != 'undefined' ) { string += "&page_name=" + page_name; }
if ( table_name != 'undefined' ) { string += "&table_name=" + table_name; }
if ( optionResult != 'undefined' ) { string += "&optionResult=" + optionResult; }
Jquery Solutions
Solution 1 - Jquery
jQuery.val() and .text() will never return 'undefined' for an empty selection. It always returns an empty string (i.e. ""). .html() will return null if the element doesn't exist though.You need to do:
if(page_name != '')
For other variables that don't come from something like jQuery.val() you would do this though:
if(typeof page_name != 'undefined')
You just have to use the typeof
operator.
Solution 2 - Jquery
if (myVariable === undefined)
or more precisely
if (typeof myVariable === 'undefined')
Note the ===
is used
Solution 3 - Jquery
function my_url (base, opt)
{
var retval = ["" + base];
retval.push( opt.page_name ? "&page_name=" + opt.page_name : "");
retval.push( opt.table_name ? "&table_name=" + opt.table_name : "");
retval.push( opt.optionResult ? "&optionResult=" + opt.optionResult : "");
return retval.join("");
}
my_url("?z=z", { page_name : "pageX" /* no table_name and optionResult */ } );
/* Returns:
?z=z&page_name=pageX
*/
This avoids using typeof whatever === "undefined"
. (Also, there isn't any string concatenation.)
Solution 4 - Jquery
http://constc.blogspot.com/2008/07/undeclared-undefined-null-in-javascript.html
Depends on how specific you want the test to be. You could maybe get away with
if(page_name){ string += "&page_name=" + page_name; }
Solution 5 - Jquery
You can just check the variable directly. If not defined it will return a falsy value.
var string = "?z=z";
if (page_name) { string += "&page_name=" + page_name; }
if (table_name) { string += "&table_name=" + table_name; }
if (optionResult) { string += "&optionResult=" + optionResult; }