javascript date validation not validation February 31

Due to what I said in the comments...

Another way you could check if a date is valid is by checking whether or not the stuff you passed into the new Date function is the same as what comes out of it, like this:

// Remember that the month is 0-based so February is actually 1...
function isValidDate(year, month, day) {
    var d = new Date(year, month, day);
    if (d.getFullYear() == year && d.getMonth() == month && d.getDate() == day) {
        return true;
    }
    return false;
}

then you could do this:

if (isValidDate(2013,1,31))

and it would return true if valid and false if invalid.

After wrecking my head with the obscurity of Date .getMonth() (and also weekday by .getDay()) being 0-index (despite year, day and all the others not being like so... oh god...) I've re-wrote Jeff's answer to make it more readable and more friendly-usable to whom consume the method from outside.

ES6 code

You can call passing month as 1-indexed as you'd normally expect.

I've parsed inputs using Number constructor so I can use strict equality to more confidently compare values.

I'm using the UTC version methods to avoid having to deal with the local timezone.

Also, I broke steps down into some variables for the sake of readability.

/**
 *
 * @param { number | string } day
 * @param { number | string } month
 * @param { number| string } year
 * @returns { boolean }
 */
function validateDateString(day, month, year) {

    day = Number(day);
    month = Number(month) - 1; //bloody 0-indexed month
    year = Number(year);

    let d = new Date(year, month, day);

    let yearMatches = d.getUTCFullYear() === year;
    let monthMatches = d.getUTCMonth() === month;
    let dayMatches = d.getUTCDate() === day;

    return yearMatches && monthMatches && dayMatches;
}