How to check whether an object is a date?

Question

I have an annoying bug in on a webpage:

date.GetMonth() is not a function

So, I suppose that I am doing something wrong. The variable date is not an object of type Date. How can I check for a datatype in Javascript? I tried to add a if (date), but it doesn't work.

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

So, if I want to write defensive code and prevent the date (which is not one) to be formatted, how do I do that?

Thanks!

UPDATE: I don't want to check the format of the date, but I want to be sure that the parameter passed to the method getFormatedDate() is of type Date.

Yakutsk

In case it should be also validated whether the date is not an Invalid Date: https://mcmap.net/q/53489/-how-to-check-whether-an-object-is-a-date – Correa

Answer 1

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]'

Sven

Out of interest do you know the reason for this failing when passing across frame boundaries? – Antipathetic

@Simon: JS globals are local to the current global object (aka window or self); different frames have their own global objects, and their properties (ie globals) refer to distinct objects: Date in frame1 is a different function object than Date in frame2; the same is true for Date.prototype, which is the reason for the instanceof failure: Date.prototype from frame1 is not part of the prototype chain of Date instances from frame2 – Sven

For elaboration on the instanceof method failing across frame boundaries, see JavaScript Garden : "... instanceof does not work on objects that originate from different JavaScript contexts (e.g. different documents in a web browser), since their constructors will not be the exact same object." – Narcolepsy

Christoph, what do you call "frame"? IFRAME, each frame in FRAMESET or something else (I mean JS-specific, not the HTML-thing)? – Emanuele

@Emanuele each HTML frame/iframe has its own JS context, and thus an own Date object. You can find all the frames in the window.frames collection, and the corresponding Date objects via window.frames[i].window.Date. For example, try window.frames[0].window.Date == Date – Kelly

@krillgar, I'd argue new Date(/anything/) is an instanceof Date. It's just an invalid instance. – Emcee

@MichaelBlackburn I agree that it is an instance of a Date object, but if it's an invalid instance, then how does that help you if you're trying to do anything constructive with it? – Alta

Detecting a Date type object (as opposed to a plain Object or a string) and validating an object you expect to be a Date are two different tasks. There are a number of situations where the input to your function could be one of a number of different data types. In my case, I can trust that any Date object I get is valid (it's not coming straight from a client) If validating is a concern, here is a post with a number of options. #1354184 – Emcee

FYI Doing something like date.setUTCHours(0) instanceof Date will return false but executing it then doing date instanceof Date returns true – Merrymaker

@Merrymaker that is because date.setUTCHours(0) does not return the date object. It returns the number of milliseconds since 1/1/1970 00:00:00 UTC. That value is a number so date.setUTCHours(0) instanceof Date will be false. – Marketplace

@Roshdy If you assing any integer to dateVariable, then (new Date(dateVariable) != "Invalid Date") will evaluate to true. Some strings also can be assigned to this variable in a way that the whole statement evaluates to true. So it doesn't work for what OP wants. – Ignatia

Object.prototype.toString.call(new Date('invalid_date')) === '[object Date]' still returns true – Commonality

'typeof date.getMonth === function' would return true if the object has a getMonth function but it is not really a date. So, there is a small risk there. – Gorlin

even invalid dates have a getMonth function. You can make a small change with: isNaN(new Date('20171212').getMonth()) if you assume a "real" date always have a month, because then it has to be a number, or it's a NaN – Parlour

Object.prototype.toString.call(date) === '[object Date]' worked well for me, thanks! – Vassily

Answer 2

You can use the following code:

(myvar instanceof Date) // returns true or false

Dionisio

Why is this not the accepted or more upvoted answer? Simply checking if date has a .getMonth property could trigger a false positive. – Beaconsfield

instanceof can trigger false negatives, see Christoph's comment to his own answer. – Sublime

@Beaconsfield Here is a demo of instanceof triggering false negative: jsbin.com/vufufoq/edit?html,js,console – Correa

Since both methods are equally flawed. You have to use @Jan's method in the debate above to make sure getMonth() doesn't return NaN on its false positive instance, at least using instanceof Date at least looks the part. – Holladay

if I do new Date("asdf") instanceof Date it's returning true. – Gean

date instanceof Date && !isNaN(date) It also checks that it is an instance of a date, and that it is a valid date (each of the tests alone is not completely reliable) – Czarism

Answer 3

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.

Correa

Wow isNaN can be used to check a Date. That's some PHP level of inanity. – Parkin

@Parkin a date is a number though. – Trickery

@Trickery Well, sure, removing all context there's a timestamp there: typeof Date.now() === "number", but: typeof new Date() === "object". More realistically, though, a date is a time and a location in space. – Parkin

This worked for me in a highly volatile environment where every possibility of string values could exist (actual dates, random letters, blanks, random characters, etc). – Kef

Answer 4

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();
}

Hueston

Check whether it's callable: if (date.getMonth && typeof date.getMonth === "function") {...} – Trust

Answer 5

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)) { // isNaN wont accept a date in typescript, use date.getTime() instead to produce a number
    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!");
}

Freetown

I'm getting this warning: "Argument of type 'Date' is not assignable to parameter of type 'number'"... Do I really need this second check? Indeed, isNaN is for numbers... – Carriole

Sounds like a typescript issue? You can use getTime() to account for this. #55427180 – Freetown

Answer 6

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.

Laudable

This worked for me, thanks. However, if you pass a single digit such as 0, or 1, it treats that as a valid Date... any thoughts? – Cellulitis

That's right, @RicardoSanchez. You probably want to use the accepted answer (Object.prototype.toString.call(value) === '[object Date]') if it's possible you'll be getting numbers. The method in this answer really tells you whether the value is convertible to a Date. – Laudable

Answer 7

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

Firewood

Answer 8

UnderscoreJS and Lodash have a function called .isDate() which appears to be exactly what you need. It's worth looking at their respective implementations: Lodash isDate, UnderscoreJs

Hillinck

Answer 9

The best way I found is:

!isNaN(Date.parse("some date test"))
//
!isNaN(Date.parse("22/05/2001"))  // true
!isNaN(Date.parse("blabla"))  // false

Counter

This doesn't work. Your true line actually is false and the question is about checking if an object is a date object... – Shoop

@Counter answer is more accurate with if a string is a date or not. That I was looking for. Thanks!! – Remittent

This is the best answer for simply checking whether a string is a date or not – Vacillation

this is not working for string with numbers (Ex -: test 1) – Pastiche

Yep, this way your system is taking 1 as YEAR and the date of 01 of January 0001 is indeed date :-) – Counter

Answer 10

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!

Gennygeno

This does not work unless you put toString() after dateVariable as an invalid date does not return a string – Sward

Answer 11

arrow function

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

Function:

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

Agler

Answer 12

I have been using a much simpler way but am not sure if this is only available in ES6 or not.

let a = {name: "a", age: 1, date: new Date("1/2/2017"), arr: [], obj: {} };
console.log(a.name.constructor.name); // "String"
console.log(a.age.constructor.name);  // "Number"
console.log(a.date.constructor.name); // "Date"
console.log(a.arr.constructor.name);  // "Array"
console.log(a.obj.constructor.name);  // "Object"

However, this will not work on null or undefined since they have no constructor.

Undeviating

Any custom made object with the constructor name "Date" returns "Date" too which is as risky as just checking if the parameter has getMonth property. – Correa

@boghyon sounds like whomever creates an object with the constructor name of a already predefined Javascript standard library is not following best practices in the first place. That would be like downloading lodash then creating your own lodash module and expecting things to work. – Undeviating

note that if the value is null then value.constructor.name throws an exception. – Flown

Answer 13

const myDate = Date.now(); // 1679252208851
const myDate2 = new Date(); // Sun Mar 19 2023 20:56:59 GMT+0200 (Восточная Европа, стандартное время)

console.log(myDate instanceof Date); // false
console.log(myDate2 instanceof Date); // true

BUT

const myDate3 = new Date(myDate); // Sun Mar 19 2023 20:56:59 GMT+0200 (Восточная Европа, стандартное время)
console.log(myDate3 instanceof Date); // true

Otes

Answer 14

In case you're using Node.js you can check if an object is a Date by using the isDate method of the util module like this.

const util = require('util');

const now = new Date();
console.log(util.types.isDate(now))

Declinatory

Answer 15

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

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

Trespass

Answer 16

Also you can use short form

function getClass(obj) {
  return {}.toString.call(obj).slice(8, -1);
}
alert( getClass(new Date) ); //Date

or something like this:

(toString.call(date)) == 'Date'

Thurnau

Answer 17

Simply use moment

import moment from 'moment';

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

Otila

Answer 18

This function will return true if it's Date or false otherwise:

function isDate(myDate) {
    return myDate.constructor.toString().indexOf("Date") > -1;
} 

Unlash

isDate(new (function AnythingButNotDate(){ })()) returns true – Correa

Answer 19

Yet another variant:

Date.prototype.isPrototypeOf(myDateObject)

Gosh

Nice and short! But unfortunately, it has the same issue as instanceof. – Correa

@BoghyonHoffmann in case of iFrame it may look like: iWindow.Date.prototype.isPrototypeOf(iWindow.date); // true iWindow.date instanceof iWindow.Date; // true – Gosh

Answer 20

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)));

Conventioneer

Answer 21

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

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

Electronics

Answer 22

I've started leaning on this function.

/**
 * @Returns Object type 
 *    - betterTypeOf(); `undefined`
 *    - betterTypeOf(null); `null`
 *    - betterTypeOf(NaN); `number`
 *    - betterTypeOf(5); `number`
 *    - betterTypeOf({}); `object`
 *    - betterTypeOf([]); `array`
 *    - betterTypeOf(''); `string`
 *    - betterTypeOf(function () {}); `function`
 *    - betterTypeOf(/a/) `regexp`
 *    - betterTypeOf(new Date()) `date`
 */
export function betterTypeOf(obj) {
  return Object.prototype.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}

And then...

if (betterTypeOf(myDateVar) === 'date' {
  console.log('it is a date')
}

Posada

Answer 23

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();
    }
}

Baird

Christoph's answer is more accurate. Having a 'call' property doesn't necessarily mean it is a function! – Hueston

Answer 24

We can also validate it by below code

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

Subtilize

The constructor of function Date() {/*...*/} is also Date. I.e. simply comparing the constructor function is too error-prone which often results in false positives. Bypass user-defined object type with https://mcmap.net/q/53489/-how-to-check-whether-an-object-is-a-date – Correa

Answer 25

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.

Harrold

This worked perfect for me. Thanks. – Cuss

Answer 26

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"
/>

Truth

Answer 27

Simplest approach of detecting an valid date.

const isDate = ( str ) => {
    let timestamp = Date.parse( str );
    if (!isNaN(timestamp)) return new Date(timestamp);
    return false
}

console.log( isDate("2020-11-11T12:12:55.123Z") )

console.log( isDate("17/July/2024") )
console.log( "getMonth: ",  isDate("17/July/2024").getMonth() )

console.log( isDate("Invalid something") )

Gaw

It looks more like a TryParse than a boolean: will new Date in your code ever return "non true" result? – Carriole

Answer 28

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

const formatDate( date: Date ) => {}

Engineman

Answer 29

Couldn't you just use

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

Klausenburg

No, only the date object has the isValid method – Darg

@grumpy @nikkwong No and no. The standard date object doesn't have isValid. Only moment.js has such an API. – Correa

isValid method is not available in date object, hence this throws up an error. – Transduction