Is this a good way to check for a valid date in JavaScript?

Question

Please correct or explain how my over-simplification is incorrect as I am not a JavaScript expert.

But I just need to know if an object is a valid date. This will o

Answer 1

Please correct or explain how my over-simplification is incorrect as I am not a JavaScript expert.

But I just need to know if an object is a valid date. This will only come from user input (ie, text box).

Here's why it's an oversimplification.

First of all, it sounds like you really want to check the validity of a string representation of a Date object. This is not particularly useful by itself, because you are going to want to use the date for something in your script, send it to the server, etc.

If you want to use the date in your script, there are caveats.

new Date('2020-10-10') // Fri Oct 09 2020 20:00:00 GMT-0400 (EDT)

If you want to pass it to the server, you'll need to do more than just check validity– you'll need to use a format that your server side code can interpret.

If that's the case, you could consider normalizing the string into a format of your choice. You'd want to be able to create equivalent dates from the normalized strings in both your client and server side code. For simplicity, the format can be human-readable (not a timestamp), and you can replace the value of the text input with the normalized string.

Checking the validity of the string can simply be a part of normalization... have the function return false or an empty string if the input was bad, don't change the text input's value, and instead show a message indicating that the value is invalid:

// assume `birthday` is a text input.

birthday.onblur = function() {
    
    var dateString = normalizeDate(birthday.value);
    
    if (dateString) {
        validator.style.display = 'none';
        birthday.value = dateString;
    } else {
        validator.style.display = 'block';
    }
        
}; 

Here's an example of what the normalizeDate function might look like. This example uses the format 'yyyy-mm-dd', you can change it to suit your needs.

function normalizeDate(dateString) {

    // If it's not at least 6 characters long (8/8/88), give up.
    if (dateString.length && dateString.length < 6) {
        return '';
    }

    var date = new Date(dateString), 
        month, day;

    // If input format was in UTC time, adjust it to local.
    if (date.getHours() || date.getMinutes()) {
        date.setMinutes(date.getTimezoneOffset());
    }
    
    month = date.getMonth() + 1;
    day = date.getDate();
    
    // Return empty string for invalid dates
    if (!day) {
        return '';
    }
    
    // Return the normalized string.
    return date.getFullYear() + '-' +
        (month > 9 ? '' : '0') + month + '-' + 
        (day > 9 ? '' : '0') + day;
}

Here's the obligatory live demo.