ECMAScript – Trailing Commas

JavaScript allowed trailing commas in array literals since the beginning. Trailing commas in other places have added in the later edition of ECMAScript.

What is trailing comma & why?

Trailing commas is also called as final commas.

It can be used while if you are adding new parameters or property to JavaScript code. If you want to add a new property, you can simply add a new line without modifying the previously last line if that line already uses a trailing comma. This makes version-control diffs cleaner and editing code might be less troublesome.

Arrays, Object literals, function parameters allow trailing commas. However, JSON doesn’t allow trailing commas.

Trailing commas in Array literals

JavaScript allows the trailing commas in arrays.

literals1

If more than one trailing comma is used, an elision (or hole) is produced. An array with holes is called sparse (a dense array has no holes). When iterating arrays for example with Array.prototype.forEach() or Array.prototype.map(), array holes are skipped.

literals2

However, the final comma in the array is ignored.

Object literals

Trailing comma is supported in Object literals from the ECMAScript 5 edition.

literals3.PNG

Trailing commas in functions

ECMAScript 2017 allows trailing commas in function parameter lists.

Trailing commas allowed function definition and function calls. Trailing commas don’t affect the length property of function declarations or their arguments object.

literals4.PNG

Function parameters with commas only are invalid which will throw SyntaxError.

literals5

What about Internet Explorer Support?

If we are using babel to convert our scripts to native JavaScript to support Internet Explorer. We don’t have to worry about the compatibility.

The babel will convert our scripts with trailing commas to support the IE.

Example: It will remove the commas while building the package.

Left side: Our ECMAScript code

Right side: Equivalent Code generated by babel

literals6.PNG

makes version-control diffs cleaner and editing code might be less troublesome

Let’s talk about the main advantage, the Version control support.

Check out the below sample, I’m trying to add an entry to list to two arrays.

One has trailing commas and other not. When we are adding an entry to an array which has trailing comma, the GIT diff looks clean. That’s the main advantage of this whole concept.

When we are having a trailing comma, it only shows the addition of entry.

gitliteral

 

And again, its based every individual to decide whether they want to use this concept or not.

Cheers,

Happy Coding!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s