How to check whether an object is a date?

As an alternative to duck typing via

typeof date.getMonth === 'function'

you can use the instanceof operator, i.e. But it will return true for invalid dates too, e.g. new Date('random_string') is also instance of Date

date instanceof Date

This will fail if objects are passed across frame boundaries.

A work-around for this is to check the object's class via

Object.prototype.toString.call(date) === '[object Date]'

You can use the following code:

(myvar instanceof Date) // returns true or false

In order to check if the value is a valid type of the standard JS-date object, you can make use of this predicate:

function isValidDate(date) {
  return date && Object.prototype.toString.call(date) === "[object Date]" && !isNaN(date);
}

1. date checks whether the parameter was not a falsy value (undefined, null, 0, "", etc..)
2. Object.prototype.toString.call(date) returns a native string representation of the given object type - In our case "[object Date]". Because date.toString() overrides its parent method, we need to .call or .apply the method from Object.prototype directly which ..
   • Bypasses user-defined object type with the same constructor name (e.g.: "Date")
   • Works across different JS contexts (e.g. iframes) in contrast to instanceof or Date.prototype.isPrototypeOf.
3. !isNaN(date) finally checks whether the value was not an Invalid Date.

The function is getMonth(), not GetMonth().

Anyway, you can check if the object has a getMonth property by doing this. It doesn't necessarily mean the object is a Date, just any object which has a getMonth property.

if (date.getMonth) {
    var month = date.getMonth();
}