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.

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]'

Answer 2

You can use the following code:

(myvar instanceof Date) // returns true or false

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.

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();
}

Answer 5

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.

Answer 6

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!");
}

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

Answer 8

The best way I found is:

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

Answer 9

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

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!

Answer 11

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.

Answer 12

arrow function

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

Function:

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

Answer 13

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

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

Answer 14

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'

Answer 15

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

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

Answer 16

Yet another variant:

Date.prototype.isPrototypeOf(myDateObject)

Answer 17

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

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

Answer 18

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)));

Answer 19

Simply use moment

import moment from 'moment';

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