[jquery] Whether a variable is undefined

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.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.

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.)

if (var === undefined)

or more precisely

if (typeof var === 'undefined')

Note the === is used

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; }

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; }