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]'

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();
}

As indicated above, it's probably easiest to just check if the function exists before using it. If you really care that it's a Date, and not just an object with a getMonth() function, try this:

function isValidDate(value) {
    var dateWrapper = new Date(value);
    return !isNaN(dateWrapper.getDate());
}

This will create either a clone of the value if it's a Date, or create an invalid date. You can then check if the new date's value is invalid or not.

This is a pretty simple approach if you're not concerned about iframes / other contexts.

// isNaN(Invalid Date) == true
if (date instanceof Date && !isNaN(date)) {
    console.log("is date!");
}

• Checks if object is actually a Date and not something that looks like one. Any object could have a getMonth function.
• Ensures the Date is not an Invalid Date
• Doesn't pass a value into new Date() where a number or even a string could be turned into a Date.

If you need to support iframes and different contexts you can use the accepted answer but add an extra check to identify invalid dates.

// isNaN(Invalid Date) == true
if (Object.prototype.toString.call(date) === '[object Date]' && !isNaN(date)) {
    console.log("is date!");
}

For all types I cooked up an Object prototype function. It may be of use to you

Object.prototype.typof = function(chkType){
      var inp        = String(this.constructor),
          customObj  = (inp.split(/\({1}/))[0].replace(/^\n/,'').substr(9),
          regularObj = Object.prototype.toString.apply(this),
          thisType   = regularObj.toLowerCase()
                        .match(new RegExp(customObj.toLowerCase()))
                       ? regularObj : '[object '+customObj+']';
     return chkType
            ? thisType.toLowerCase().match(chkType.toLowerCase()) 
               ? true : false
            : thisType;
}

Now you can check any type like this:

var myDate     = new Date().toString(),
    myRealDate = new Date();
if (myRealDate.typof('Date')) { /* do things */ }
alert( myDate.typof() ); //=> String

[Edit march 2013] based on progressing insight this is a better method:

Object.prototype.is = function() {
        var test = arguments.length ? [].slice.call(arguments) : null
           ,self = this.constructor;
        return test ? !!(test.filter(function(a){return a === self}).length)
               : (this.constructor.name ||
                  (String(self).match ( /^function\s*([^\s(]+)/im)
                    || [0,'ANONYMOUS_CONSTRUCTOR']) [1] );
}
// usage
var Some = function(){ /* ... */}
   ,Other = function(){ /* ... */}
   ,some = new Some;
2..is(String,Function,RegExp);        //=> false
2..is(String,Function,Number,RegExp); //=> true
'hello'.is(String);                   //=> true
'hello'.is();                         //-> String
/[a-z]/i.is();                        //-> RegExp
some.is();                            //=> 'ANONYMOUS_CONSTRUCTOR'
some.is(Other);                       //=> false
some.is(Some);                        //=> true
// note: you can't use this for NaN (NaN === Number)
(+'ab2').is(Number);                 //=> true

Instead of all the workarounds you can use the following:

dateVariable = new Date(date);
if (dateVariable == 'Invalid Date') console.log('Invalid Date!');

I found this hack better!

arrow function

const isValidDate = (value: any) => value instanceof Date && !isNaN(value);

Function:

function isValidDate(d) {
  return d instanceof Date && !isNaN(d);
}

You could check if a function specific to the Date object exists:

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

An approach using a try/catch

function getFormattedDate(date = new Date()) {
  try {
    date.toISOString();
  } catch (e) {
    date = new Date();
  }
  return date;
}

console.log(getFormattedDate());
console.log(getFormattedDate('AAAA'));
console.log(getFormattedDate(new Date('AAAA')));
console.log(getFormattedDate(new Date(2018, 2, 10)));

with the following approach, you can even check date no to be "Invalid Date"

if(!!date.getDate()){
    console.log('date is valid')
}

Simply use moment

import moment from 'moment';

moment(myvar).isValid(); //  return true or false

Actually date will be of type Object. But you can check if the object has getMonth method and if it is callable.

function getFormatedDate(date) {
    if (date && date.getMonth && date.getMonth.call) {
       var month = date.getMonth();
    }
}

We can also validate it by below code

var a = new Date();
a.constructor === Date
/*
true
*/

Inspired by this answer, this solution works in my case(I needed to check whether the value recieved from API is a date or not):

!isNaN(Date.parse(new Date(YourVariable)))

This way, if it is some random string coming from a client, or any other object, you can find out if it is a Date-like object.

I had some issues with React hooks where the Date would come in later / lazy loaded and then the initial state can't be null, it won't pass ts checks, but apparently an empty Object does the trick then! :)

const [birthDate, setBirthDate] = React.useState({})

<input
  value={birthDate instanceof Date ? birthDate.toISOString() : ''}
  name="birthDay"
/>

If you are using Typescript you could check using the Date type:

const formatDate( date: Date ) => {}