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]'