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`");
}
Here's how I would do it:
if (Object.prototype.toString.call(d) === "[object Date]") {
// it is a date
if (isNaN(d.getTime())) { // d.valueOf() could also work
// date is not valid
} else {
// date is valid
}
} else {
// not a date
}
Update [2018-05-31]: If you are not concerned with Date objects from other JS contexts (external windows, frames, or iframes), this simpler form may be preferred:
function isValidDate(d) {
return d instanceof Date && !isNaN(d);
}
Best Answer
The problem is autofill is handled differently by different browsers. Some dispatch the change event, some don't. So it is almost impossible to hook onto an event which is triggered when browser autocompletes an input field.
Change event trigger for different browsers:
For username/password fields:
For other form fields:
You best options are to either disable autocomplete for a form using
autocomplete="off"
in your form or poll at regular interval to see if its filled.For your question on whether it is filled on or before document.ready again it varies from browser to browser and even version to version. For username/password fields only when you select a username password field is filled. So altogether you would have a very messy code if you try to attach to any event.
You can have a good read on this HERE