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

User · Answer

You can use the following code    myvar instanceof Date     returns true or false

User · Answer

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

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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

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This is a pretty simple approach and doesn t experience a lot of the edge cases in the existing answers     Invalid Date getTime   will produce NaN if  date instanceof Date  amp  amp  date getTime          console log  quot is date  quot       It won t fire with other objects like numbers  makes sure the value is actually a Date  rather than an object that looks like one   and it avoids Invalid Dates

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Yet another variant    Date prototype isPrototypeOf myDateObject

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We can also validate it by below code  var a   new Date    a constructor     Date    true

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You could check if a function specific to the Date object exists   function getFormatedDate date        if  date getMonth            var month   date getMonth

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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

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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

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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

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The best way I found is    isNaN Date parse  some date test        isNaN Date parse  22 05 2001        true  isNaN Date parse  blabla        false

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This function will return true if it s Date or false otherwise   function isDate myDate        return myDate constructor toString   indexOf  Date    gt  -1

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If you are using Typescript you could check using the Date type  const formatDate  date  Date     gt

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Couldn t you just use   function getFormatedDate date        if  date isValid             var month   date GetMonth

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An approach using a try catch   x000D   x000D  function getFormatedDate date   new Date      x000D    try   x000D      date toISOString    x000D      catch  e    x000D      date   new Date    x000D      x000D    return date  x000D    x000D   x000D  console log getFormatedDate     x000D  console log getFormatedDate  AAAA     x000D  console log getFormatedDate new Date  AAAA      x000D  console log getFormatedDate new Date 2018  2  10     x000D   x000D   x000D

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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

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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  amp  amp  Object prototype toString call date        object Date    amp  amp   isNaN date        date checks whether the parameter was not a falsy value  undefined  null  0      etc    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    isNaN date  finally checks whether the value was not an Invalid Date

User · Answer

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

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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

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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  amp  amp  date getMonth  amp  amp  date getMonth call           var month   date getMonth

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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          gt  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              gt  false 2  is String Function Number RegExp       gt  true  hello  is String                         gt  true  hello  is                              - gt  String   a-z  i is                             - gt  RegExp some is                                   gt   ANONYMOUS CONSTRUCTOR  some is Other                             gt  false some is Some                              gt  true    note  you can t use this for NaN  NaN     Number     ab2   is Number                       gt  true

[javascript] How to check whether an object is a date?

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