If you really want to check for null, do:<br /><br />    if (null == yourvar) // with casting<br />    if (null === yourvar) // without casting<br /><br /><br />If you want to check if a variable exist<br /><br />    if (typeof yourvar != 'undefined') // Any scope<br />    if (window['varname'] != undefined) // Global scope<br />    if (window['varname'] != void 0) // Old browsers<br /><br /><br />If you know the variable exists but don't know if there's any value<br />stored in it:<br /><br />    if (undefined != yourvar)<br />    if (void 0 != yourvar) // for older browsers<br /><br /><br />If you want to know if a member exists independent of whether it has<br />been assigned a value or not:<br /><br />    if ('membername' in object) // With inheritance<br />    if (object.hasOwnProperty('membername')) // Without inheritance<br /><br /><br />If you want to to know whether a variable autocasts to true:<br /><br />    if(variablename)<br />