How to know if an object is a Date or not with Javascript?

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 since somewhere and the object 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

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 a getMonth property.

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

Answer 2

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

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

Answer 3

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 4

As an alternative to duck typing via

typeof date.getMonth === 'function'

you can use the instanceof operator, i.e.

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 5

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();
    }
}

Answer 6

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