The Date.now() function in JavaScript returns the current timestamp in milliseconds since the Unix Epoch (January 1, 1970, 00:00:00 UTC). Unlike the new Date() constructor, Date.now() returns a number, not a Date object.

Syntax:

const currentTimestamp = Date.now();

Explanation:

• Date.now() gives the number of milliseconds that have passed since January 1, 1970.
• This is often used to measure time intervals or calculate time differences.

Example:

const timestamp = Date.now();
console.log(timestamp);

Output:

1729570415123

Explanation:

• The output will be a large number representing the current timestamp in milliseconds (this will differ depending on when the code is executed).

Use Cases:

1. Calculating Time Differences

You can use Date.now() to measure how long an operation takes by storing the current timestamp at the start and end of the operation.

const start = Date.now();

// Simulate some operation (e.g., a delay)
for (let i = 0; i < 1000000000; i++) {}

const end = Date.now();
console.log(`Time taken: ${end - start} milliseconds`);

Output:

Time taken: 153 milliseconds

Explanation:

• Date.now() is called before and after a time-consuming operation to measure how long it took in milliseconds.

2. Creating a Unique ID Based on Time

You can use Date.now() to generate unique identifiers for events or records based on the current timestamp.

const uniqueID = `id_${Date.now()}`;
console.log(uniqueID);

Output:

id_1729570453789

Summary:

• Date.now() returns the current timestamp as the number of milliseconds since January 1, 1970.
• It is useful for time-based calculations, performance measurements, and generating unique timestamps.
• Unlike new Date(), it does not return a Date object, but instead, a raw millisecond timestamp.