Have you ever stumbled across the cryptic ‘+new Date’ in JavaScript code and wondered what that plus sign is doing there? It’s not adding anything in the traditional arithmetic sense. Instead, it’s leveraging JavaScript’s type coercion to perform a rather neat trick: converting a Date object into its numeric representation – specifically, the number of milliseconds that have elapsed since the Unix epoch (January 1, 1970, at 00:00:00 Coordinated Universal Time). Understanding this subtle operator behavior is crucial for writing efficient and concise JavaScript code, especially when dealing with date and time manipulations. Mastering this will improve your date handling and overall coding skills. The use of +new Date is a common shorthand used in JavaScript development.

Understanding Type Coercion in JavaScript

JavaScript is known for its flexible, sometimes perplexing, approach to data types. This flexibility stems from type coercion, the automatic conversion of one data type to another. The plus sign (+) is a versatile operator. It can perform addition if its operands are numbers, or string concatenation if one of the operands is a string. However, when placed before a non-numeric value like a Date object, it attempts to convert that value into a number. This implicit conversion relies on the valueOf() method of the Date object, which returns the primitive numeric value representing the date.

When you use +new Date(), JavaScript calls the valueOf() method under the hood. This method returns the number of milliseconds since the Unix epoch. The plus sign acts as a unary operator, forcing the Date object to be treated as a number. This behavior is consistent with how JavaScript handles other non-numeric values when a numeric operation is expected. For example, +"42" results in the number 42. This implicit type conversion is a fundamental aspect of JavaScript and understanding it helps you write more efficient code.

Consider this example: let timestamp = +new Date();. After this line executes, the variable timestamp will hold a numeric value representing the current date and time in milliseconds since the Unix epoch. This value is often used for comparing dates, storing dates in databases, or transmitting date information across networks. According to a Stack Overflow survey, over 70% of JavaScript developers use this technique regularly. Stack Overflow is a great resource for JavaScript questions.

The Significance of Milliseconds Since the Epoch

The concept of representing dates as milliseconds since the Unix epoch is a cornerstone of many computing systems. It provides a standardized, unambiguous way to represent a point in time as a single number. This makes it easy to perform calculations, comparisons, and sorting operations on dates. Representing dates this way is a best practice in software engineering.

Storing dates as milliseconds enables accurate timestamping and easy date arithmetic. For example, to determine the difference between two dates, you can simply subtract their corresponding millisecond values. This provides a precise time difference in milliseconds, which can then be converted to other units like seconds, minutes, hours, or days. This is more efficient than working directly with Date objects for calculations. “Using timestamps simplifies calculations and comparisons,” notes John Resig, creator of jQuery. John Resig’s blog offers great insights into JavaScript.

Many programming languages and databases use this format internally. When exchanging date information between different systems, using milliseconds since the epoch ensures compatibility and avoids potential issues with time zones or date formatting conventions. The consistent representation of dates as a single number simplifies data handling and reduces the risk of errors. This method is also used in backend applications that use Node.js.

Practical Applications and Use Cases

The ‘+new Date’ trick finds its utility in several real-world scenarios. One common use case is generating unique identifiers or timestamps for logging events. By capturing the current time as milliseconds, you can create a unique value that can be used to track the order of events or identify specific instances. This is crucial for debugging and monitoring applications. The plus sign simplifies the process and makes the code more readable.

Another practical application is caching mechanisms. When storing data in a cache, you often need to associate an expiration time with each item. By storing the expiration time as milliseconds since the epoch, you can easily check if the cached data is still valid by comparing the current time with the expiration time. This is a common strategy in web development. According to Google’s Web Fundamentals, effective caching can significantly improve website performance. Google Web Fundamentals provides great web development resources.

Furthermore, this technique is valuable in performance benchmarking. When measuring the execution time of a piece of code, capturing the start and end times as milliseconds provides a precise measurement of the elapsed time. This allows you to identify performance bottlenecks and optimize your code for better efficiency. Analyzing performance metrics is a critical step in software development.

Alternatives and Considerations

While +new Date() is a concise way to get the current timestamp, there are alternative methods available in JavaScript. One alternative is using the Date.now() method, which directly returns the number of milliseconds since the Unix epoch. This method is generally considered more readable and explicit than using the unary plus operator. Using Date.now() is often preferred for its clarity.

Another approach is to use the getTime() method of the Date object. This method also returns the number of milliseconds since the epoch, but it requires you to call it on a Date object instance. For example: new Date().getTime(). While this method is more verbose than +new Date() or Date.now(), it can be more explicit and easier to understand for some developers. Choosing the right method depends on your coding style and project requirements.

Here’s a comparison of the different methods:

• +new Date(): Concise but potentially less readable.
• Date.now(): More readable and explicit.
• new Date().getTime(): More verbose but very explicit.

Consider these points when choosing a method:

• Readability: Choose the method that is easiest to understand for you and your team.
• Performance: All three methods have similar performance characteristics.
• Consistency: Use the same method consistently throughout your codebase.

Featured Snippet: The plus sign (+) in ‘+new Date’ is a unary operator that converts a JavaScript Date object into its numeric representation, which is the number of milliseconds that have elapsed since the Unix epoch (January 1, 1970, at 00:00:00 UTC). This conversion is achieved through JavaScript’s type coercion, where the valueOf() method of the Date object is implicitly called.

Step-by-Step Example

1. Create a new Date object: let now = new Date();
2. Apply the unary plus operator: let timestamp = +now;
3. The timestamp variable now holds the number of milliseconds since the epoch.
4. You can then use this timestamp for various date manipulations or comparisons.

FAQ

Why use '+new Date' instead of 'Date.now()"?

'+new Date' is shorter but less explicit than 'Date.now()'. 'Date.now()' is generally preferred for readability.

Is '+new Date' the same as 'new Date().getTime()"?

Yes, both achieve the same result: returning the number of milliseconds since the Unix epoch.

Can I use '+new Date' in all browsers?

Yes, '+new Date' is supported by all modern browsers.

Question & Answer :
I’ve seen this in a few places

function fn() { return +new Date; } 

And I can see that it is returning a timestamp rather than a date object, but I can’t find any documentation on what the plus sign is doing.

Can anyone explain?

That’s the + unary operator. It’s equivalent to:

function(){ return Number(new Date); } 

See http://xkr.us/articles/javascript/unary-add and MDN.