The usual way to check if the value of a property is the special value undefined, is:
if(o.myProperty === undefined) {
alert("myProperty value is the special value `undefined`");
}
To check if an object does not actually have such a property, and will therefore return undefined by default when you try and access it:
if(!o.hasOwnProperty('myProperty')) {
alert("myProperty does not exist");
}
To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined) identifier without an early error:
if(typeof myVariable === 'undefined') {
alert('myVariable is either the special value `undefined`, or it has not been declared');
}
In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.
However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.
If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined value itself:
if(myVariable === void 0) {
alert("myVariable is the special value `undefined`");
}
In JavaScript, everything is 'truthy' or 'falsy', and for numbers 0 means false, everything else true. So you could write:
if ($(selector).length)
You don't need that >0 part.
Best Answer
Try
An undefined variable cannot be referred directly. However, all global variables are attributes of the same name of the global context (
windowin case of browsers), and accessing an undefined attribute is fine.Or use
if (typeof console === 'undefined') console = ...if you want to avoid the magic variablewindow, see @Tim Down's answer.