How Can I Reliably Validate JavaScript Date Instances?

Validating Date Instance Validity in JavaScript

Determining the validity of Date instances in JavaScript can be challenging. Let's explore the "invalid date" problem and find a solution to reliably detect invalid Date objects:

The Date object is a JavaScript construct that represents a specific instant in time. Issues arise when creating Date instances from invalid date strings, as shown in the example below:

var d = new Date("foo");

This code produces an "Invalid Date" instance, which is of type object and inherits from the Date prototype. However, Object.prototype.toString.call(d) returns "[object Date]", and d instanceof Date evaluates to true.

Solution:

Here's a straightforward approach to check for valid Date instances:

if (Object.prototype.toString.call(d) === "[object Date]") {
  if (isNaN(d.getTime())) {
    // Invalid Date
  } else {
    // Valid Date
  }
} else {
  // Not a Date object
}

This method ensures that the instance is a Date object and that its time value is not NaN. If either condition fails, the Date instance is considered invalid.

Update [2018-05-31]:

For cases where Date objects from external contexts are not a concern, a simpler validation function can be used:

function isValidDate(d) {
  return d instanceof Date && !isNaN(d);
}

Note [2021-02-01]:

It's important to note that there is a distinction between "invalid dates" and "invalid date objects". This answer focuses on validating Date instances, not date input strings.

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