Thursday, 20 December 2012

jQuery and Javascript


Here I am adding some memory nudges about javascript and jQuery.


Check for variable is defined


in JavaScript null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.
Second, no, they are not directly equivalent. If you really want to check for null, do:
if (null == yourvar) // with casting
if (null === yourvar) // without casting
If you want to check if a variable exist
if (typeof yourvar != 'undefined') // Any scope
if (window['varname'] != undefined) // Global scope
if (window['varname'] != void 0) // Old browsers
If you know the variable exists but don't know if there's any value stored in it:
if (undefined != yourvar)
if (void 0 != yourvar) // for older browsers
If you want to know if a member exists independent of whether it has been assigned a value or not:
if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance
If you want to to know whether a variable autocasts to true:
if(variablename)
I probably forgot some method as well...

Taken from: http://stackoverflow.com/questions/858181/how-to-check-a-not-defined-variable-in-javascript