Check if JavaScript string represents Date

Question

I am stuck with checking if the string represents Date. Currently I have a situation where I need to check if "Tue May 16 2017 00:00:00 GMT+0400 (Georgian Standard Time)" this string represents Date. I wrote the following code:

var tryConvert = new Date(input);
var tryMonth = tryConvert.getMonth();
if (!(tryMonth !== tryMonth)) {//checking for NaN
    return true;
}

The problem is that input can be integer 96 and it successfully returns true. The desired behavior is that it should work only for Date instances, ISO strings /\d\d\d\d\-\d\d\-\d\dT\d\d\:\d\d\:\d\d/g and Date.toString() strings like "Tue May 16 2017 00:00:00 GMT+0400 (Georgian Standard Time)". How can I achieve this?

To make it more clear here is the full code:

function isDate(input) {
    if (!input) {
        return false;
    }

    if (input instanceof Date) {
        return true;
    }

    var rx = /\d\d\d\d\-\d\d\-\d\dT\d\d\:\d\d\:\d\d/g;
    var time = rx.exec(input);

    if (time) {
        return true;
    }

    if (typeof input === 'string' || input instanceof String) {
        var tryConvert = new Date(input);
        var tryMonth = tryConvert.getMonth();
        if (!(tryMonth !== tryMonth)) {
            return true;
        }
    }

    return false;
};

Possible duplicate of [Check if a string is a date value](https://stackoverflow.com/questions/7445328/check-if-a-string-is-a-date-value) — Zenoo, May 30 '17 at 13:34
moment.js is a nice lib for dates handling that has a parse method for a specific pattern. Maybe it could help? — Luan Nico, May 30 '17 at 13:34
@SLaks, not sure what will be (new Date).toString() for user from US for example. How can I create a good regex for it? — Giorgi Nakeuri, May 30 '17 at 13:37
Here is another thread with some solutions: [Detecting an “invalid date” Date instance in JavaScript](https://stackoverflow.com/questions/1353684/detecting-an-invalid-date-date-instance-in-javascript) — UfguFugullu, May 30 '17 at 13:41
Would using an `isNaN(input)` check in combination with your `typeof input === 'string' || input instanceof String` check accomplish what you need? `isNaN()` would be able to distinguish between the `96` string and an actual date string. — Lennholm, May 30 '17 at 13:45
@MikaelLennholm, no I have tried it. `new Date("5555");` is a valid date :( — Giorgi Nakeuri, May 30 '17 at 13:49
@GiorgiNakeuri Yes, but I mean if you return false if `!isNaN(input)` without even creating a date. `isNaN("5555") === false` but `isNaN("Tue May 16 2017 00:00:00 GMT+0400 (Georgian Standard Time)") === true` — Lennholm, May 30 '17 at 13:55
@MikaelLennholm, you are my hero today! Please add your comment as an answer! — Giorgi Nakeuri, May 30 '17 at 14:05

score 1 · Accepted Answer · answered May 30 '17 at 14:18

For your use case, combining your check if input is a string with a check if it's a parseable number or not should do the trick.

Here I've added added this check where you check if input is a string:

if ((typeof input === 'string' || input instanceof String) && isNaN(input)) {
    var tryConvert = new Date(input);
    var tryMonth = tryConvert.getMonth();
    if (!(tryMonth !== tryMonth)) {
        return true;
    }
}

return false;

Basically, this returns false if input is a string containing 96, 5555 or any other parseable number but actual date strings such as Tue May 16 2017 00:00:00 GMT+0400 (Georgian Standard Time) are not parseable numbers so they will pass the isNaN() check and proceed to the conversion test.

You can probably remove the RegExp test as well since it only tests one particular date format.

Check if JavaScript string represents Date

1 Answers1