JavaScript: 2 Ways to Clone (Deep Copy) a Date Object

In JavaScript, Date objects are mutable, so if you simply assign a Date object to a new variable, changes to the new variable will affect the original Date object. This article will walk you through a couple of different ways to create a truly new Date object that is a clone (or deep copy) of an existing Date object.

Using the Date() constructor

You can use the Date constructor to create a new Date object with the same value as the original Date object.

Example:

const originalDate = new Date('2023-03-07T12:30:00.000Z');
const clonedDate = new Date(originalDate);

// Modify clonedDate
clonedDate.setFullYear(2030);
clonedDate.setDate(8);

console.log("Original date:", originalDate); 
console.log("Cloned date:", clonedDate); 

Output:

Original date: 2023-03-07T12:30:00.000Z
Cloned date: 2030-03-08T12:30:00.000Z

Using the getTime() method

The getTime() method can help us create a new Date object with the same timestamp value as the original Date object.

Example:

const originalDate = new Date("2023-01-01T12:00:00.000Z");

// Get the timestamp
const timestamp = originalDate.getTime();

// Clone the date
const clonedDate = new Date(timestamp);

// Modify clonedDate
clonedDate.setFullYear(2024);
clonedDate.setMonth(11);

console.log("Original date:", originalDate); 
console.log("Cloned date:", clonedDate);

Output:

Original date: 2023-01-01T12:00:00.000Z
Cloned date: 2024-12-01T12:00:00.000Z

What about using JSON.stringify() and JSON.parse()?

When you use JSON.stringify() on a Date object, it will convert the Date object into a string representation of the date. However, when you parse the string back into an object using JSON.parse(), it will not be a Date object but rather a string. Let me prove this fact with the code below:

const date = new Date("2029-01-01T00:00:00.000Z");

const json = JSON.stringify(date);

const clonedDate = JSON.parse(json);

console.log(clonedDate instanceof Date); 
console.log(typeof clonedDate); 

Output:

false
string

Given the above, it is better NOT to use JSON.stringify() to deep copy a Date object. If you still want to, you can do the following (it’s inefficient and too verbose):

// Original Date
const originalDate = new Date("2023-06-01T00:00:00.000Z");

// Clone Date
const clonedDate = new Date(JSON.parse(JSON.stringify(originalDate)));

// Modify the cloned date
clonedDate.setFullYear(2024);

// Log the original date
console.log("Original Date: ", originalDate);

// Log the cloned date
console.log("Cloned Date: ", clonedDate);

Output:

Original Date:  2023-06-01T00:00:00.000Z
Cloned Date:  2024-06-01T00:00:00.000Z

Conclusion

You’ve learned some techniques to deep copy a Date object in JavaScript. This knowledge can be helpful in some situations. For example, when implementing undo/redo functionality in an app, it may be necessary to clone Date objects in order to maintain the state of the application at different points in time.