javascript - How to check whether an object is a date? - Stack Overflow

admin管理员组

文章数量:1125489

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.

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.

• javascript
• date

Share Improve this question Follow edited Aug 21, 2018 at 15:25 Alexander Abakumov 14.5k1616 gold badges9696 silver badges132132 bronze badges asked Mar 13, 2009 at 17:31 MartinMartin 40.5k6565 gold badges199199 silver badges285285 bronze badges 1

• In case it should be also validated whether the date is not an Invalid Date: stackoverflow.com/a/44198641/5846045 – Boghyon Hoffmann Commented Apr 11, 2020 at 11:45

Add a comment  | 

28 Answers 28

Sorted by: Reset to default 1561

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]'

Be aware, that the instanceof solution doesn't work when using multiple realms:

JavaScript execution environments (windows, frames, etc.) are each in their own realm. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, [] instanceof window.frames[0].Array will return false, because Array.prototype !== window.frames[0].Array.prototype and arrays in the current realm inherit from the former.

You can use the following code:

(myvar instanceof Date) // returns true or false

Be aware, that this solution doesn't work when using multiple realms:

JavaScript execution environments (windows, frames, etc.) are each in their own realm. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, [] instanceof window.frames[0].Array will return false, because Array.prototype !== window.frames[0].Array.prototype and arrays in the current realm inherit from the former.

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();
}

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!");
}

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.

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

The best way I found is:

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

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

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!

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

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.

arrow function

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

Function:

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

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))

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

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

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'

Simply use moment

import moment from 'moment';

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

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')
}

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

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

Yet another variant:

Date.prototype.isPrototypeOf(myDateObject)

We can also validate it by below code

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

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

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

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)));

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();
    }
}

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"
/>

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") )

Couldn't you just use

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

本文标签： javascriptHow to check whether an object is a dateStack Overflow