JavaScript - detecting invalid date

In this article, we're going to have a look at how to check if data is invalid in JavaScript.

Date correctness can be checked basing on internal state of Date object.

Incorrect date object returns:

• false for call isFinite(myDate),
• true for call isNaN(myDate),
• NaN from myDate.getTime() method,
• "Invalid Date" string for toString() method call.

Quick solution:

// ONLINE-RUNNER:browser;

var correctDate   = new Date(2020, 10, 15);
var incorrectDate = new Date(NaN);

//                                           true  - means correct
//                                           false - means incorrect
//
console.log( isFinite( correctDate   ) ); // true
console.log(   !isNaN( correctDate   ) ); // true
console.log( isFinite( incorrectDate ) ); // false
console.log(   !isNaN( incorrectDate ) ); // false

Look below to see different practical examples:

1. Date object test example

This approach is based on fact: isFinite method returns false for incorrect dates.

// ONLINE-RUNNER:browser;

function isCorrectDate(date) {
    return date instanceof Date && isFinite(date);

    // return date instanceof Date && !isNaN(date);
}

// Usage example:

var correctDate   = new Date(2020, 10, 15);
var incorrectDate = new Date(NaN);

console.log( isCorrectDate( correctDate   ) ); // true
console.log( isCorrectDate( incorrectDate ) ); // false

2. getTime() or other getter test example

This approach is based on fact: incorrect date returns NaN values from getters. So we can make a simple test for example with getTime() method.

// ONLINE-RUNNER:browser;

function isCorrectDate(date) {
    return date instanceof Date && isFinite(date.getTime());

    // return date instanceof Date && !isNaN(date.getTime());
}

// Usage example:

var correctDate   = new Date(2020, 10, 15);
var incorrectDate = new Date(NaN);

console.log( isCorrectDate( correctDate   ) ); // true
console.log( isCorrectDate( incorrectDate ) ); // false

3. toString() method test example

This approach is based on fact: incorrect date object returns "Invalid Date" string for toString() method call. It is good to call to string method with protoype API that prevents methods overriding.

// ONLINE-RUNNER:browser;

function isCorrectDate(date) {
    if (date instanceof Date) {
        var text = Date.prototype.toString.call(date);
        return text !== 'Invalid Date';
    }
    return false;
}

// Usage example:

var correctDate   = new Date(2020, 10, 15);
var incorrectDate = new Date(NaN);

console.log( isCorrectDate( correctDate   ) ); // true
console.log( isCorrectDate( incorrectDate ) ); // false

4. Detecting correct date string or date object example

The example presented in this section allows checking the correctness of the date provided as a string or object. String date should be formatted according to one of the standards:

• IETF-compliant RFC 2822 timestamps,
• ISO 8601.

Run the following code to see the effect:

// ONLINE-RUNNER:browser;

function isCorrectDate(date) {
    return isFinite(date instanceof Date ? date : new Date(date));

    // return isFinite(date instanceof Date ? date : Date.parse(date));
    // return !isNaN(date instanceof Date ? date : new Date(date));
    // return !isNaN(date instanceof Date ? date : Date.parse(date));
}

// Usage example:

var date1 = '2020-10-15';
var date2 = '2020-10-x5'; // <-------- incorrect date
var date3 = 'This is not date'; // <-- incorrect date
var date4 = new Date(2020, 10, 15);
var date5 = new Date(NaN); // <------- incorrect date

console.log( isCorrectDate( date1 ) ); // true
console.log( isCorrectDate( date2 ) ); // false
console.log( isCorrectDate( date3 ) ); // false
console.log( isCorrectDate( date4 ) ); // true
console.log( isCorrectDate( date5 ) ); // false