How to Determine if a Variable is a Date
JavaScript dates are considered objects. The
typeof operator returns
'object' for dates, so you can't use typeof to distinguish whether
a value is a date. You should use instanceof instead.
let x = new Date();
if (x instanceof Date) {
// will execute
}Object.prototype.toString.call()
A longer alternative is the Object.prototype.toString.call(variableToCheck) method.
It returns the internal class property of an object in a string of format '[object Type]',
so a date would be '[object Date]'.
let x = new Date();
if (Object.prototype.toString.call(x) === "[object Date]") {
// will execute
}Check for validity
Now that you know how to check if the variable is a date,
you need to check if it is a valid date. You can use the
!isNaN() function to check whether a date is valid.
let x = new Date("Bad String");
if (x instanceof Date) {
// executes, because `x` is technically a date object
}
if (x instanceof Date && !isNaN(x)) {
// will not execute
}If x is a Date, isNaN(x) is equivalent to Number.isNaN(x.valueOf()).
Dates have a valueOf() function that returns a numeric representation of the date as milliseconds since the Unix epoch.
// 86400000, or 24 * 60 * 60 * 1000
new Date('1970-01-02T00:00:00.000Z').valueOf();Another common trick you might see is using the > operator. Remember that you can use < and > to compare dates in JavaScript as shown below, so d > 0 will return true if d is a date with a positive, non-NaN valueOf().
const validDate = new Date('2021-01-01');
const zeroDate = new Date(0);
const invalidDate = new Date('fail');
validDate > 0; // true
zeroDate > 0; // false
invalidDate > 0; // false
validDate >= 0; // true
zeroDate >= 0; // true
invalidDate >= 0; // false