How to validate date with format "mm/dd/yyyy" in JavaScript?

Question

I want to validate the date format on an input using the format mm/dd/yyyy.

I found below codes in one site and then used it but it doesn't work:

function isDate(ExpiryDate) { 
    var objDate,  // date object initialized from the ExpiryDate string 
        mSeconds, // ExpiryDate in milliseconds 
        day,      // day 
        month,    // month 
        year;     // year 
    // date length should be 10 characters (no more no less) 
    if (ExpiryDate.length !== 10) { 
        return false; 
    } 
    // third and sixth character should be '/' 
    if (ExpiryDate.substring(2, 3) !== '/' || ExpiryDate.substring(5, 6) !== '/') { 
        return false; 
    } 
    // extract month, day and year from the ExpiryDate (expected format is mm/dd/yyyy) 
    // subtraction will cast variables to integer implicitly (needed 
    // for !== comparing) 
    month = ExpiryDate.substring(0, 2) - 1; // because months in JS start from 0 
    day = ExpiryDate.substring(3, 5) - 0; 
    year = ExpiryDate.substring(6, 10) - 0; 
    // test year range 
    if (year < 1000 || year > 3000) { 
        return false; 
    } 
    // convert ExpiryDate to milliseconds 
    mSeconds = (new Date(year, month, day)).getTime(); 
    // initialize Date() object from calculated milliseconds 
    objDate = new Date(); 
    objDate.setTime(mSeconds); 
    // compare input date and parts from Date() object 
    // if difference exists then date isn't valid 
    if (objDate.getFullYear() !== year || 
        objDate.getMonth() !== month || 
        objDate.getDate() !== day) { 
        return false; 
    } 
    // otherwise return true 
    return true; 
}

function checkDate(){ 
    // define date string to test 
    var ExpiryDate = document.getElementById(' ExpiryDate').value; 
    // check date and print message 
    if (isDate(ExpiryDate)) { 
        alert('OK'); 
    } 
    else { 
        alert('Invalid date format!'); 
    } 
}

Any suggestion about what could be wrong?

Answer 1

I think Niklas has the right answer to your problem. Besides that, I think the following date validation function is a little bit easier to read:

// Validates that the input string is a valid date formatted as "mm/dd/yyyy"
function isValidDate(dateString)
{
    // First check for the pattern
    if(!/^\d{1,2}\/\d{1,2}\/\d{4}$/.test(dateString))
        return false;

    // Parse the date parts to integers
    var parts = dateString.split("/");
    var day = parseInt(parts[1], 10);
    var month = parseInt(parts[0], 10);
    var year = parseInt(parts[2], 10);

    // Check the ranges of month and year
    if(year < 1000 || year > 3000 || month == 0 || month > 12)
        return false;

    var monthLength = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];

    // Adjust for leap years
    if(year % 400 == 0 || (year % 100 != 0 && year % 4 == 0))
        monthLength[1] = 29;

    // Check the range of the day
    return day > 0 && day <= monthLength[month - 1];
};