How to check if a variable is a date in JavaScript

Checking whether a variable is a date in JavaScript is not inherently difficult, but there are certain nuances and challenges that can arise due to the flexibility and dynamic nature of the language.

Using typeof might not always provide the expected result. In this case, typeof returns “object” which doesn’t specifically indicate that the variable is a Date object.

const now = new Date();
console.log(typeof now);  // "object"

Also just because a variable is a date object doesn’t guarantee it’s a valid date. You may need further checks for valid date ranges or formats.

Using instanceof operator

The instanceof operator is a powerful tool for determining whether an object is an instance of a specific class or constructor function. It tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object.

function isDate(variable) {
  return variable instanceof Date;
}

const myDate = new Date();
console.log(isDate(myDate)); // true

const myNumber = 2024;
console.log(isDate(myNumber)); // false

const myString = "Hello";
console.log(isDate(myString)); // false

When working with iframes or different JavaScript environments (realms), instanceof might not behave consistently. This is because the instanceof relies on the prototype chain, and objects created in different realms may have different prototypes even if they represent the same built-in type.

This scenario is not a common situation in typical web development. It might occur in more complex setups involving iframes, web workers, or other advanced use cases where multiple JavaScript environments are in play.

Using Object.prototype.toString.call()

You can use the Object.prototype.toString.call() method to get the internal class of a variable, and compare it with the string "[object Date]". This method is more reliable than the instanceof operator, as it works across different contexts.

function isDate(variable) {
  return Object.prototype.toString.call(variable) === '[object Date]';
}

const myDate = new Date();
console.log(isDate(myDate)); // true

const myNumber = 2024;
console.log(isDate(myNumber)); // false

const myString = "Hello";
console.log(isDate(myString)); // false

This approach is more robust and handles instances from different worlds, but it might be slightly less performant than instanceof.