DEV Community

Scott Chamberlain
Scott Chamberlain

Posted on • Originally published at recology.info on

trailing commas

Let’s talk about trailing commas (aka: “final commas”, “dangling commas”). Trailing commas refers to a comma at the end of a series of values in an array or array like object, leaving an essentially empty slot. e.g., [1, 2, 3,]

I kind of like them when I work on Ruby and Python projects. A number of advantages of trailing commas have been pointed out, the most common of which is diffs:

diff --git a/hello.json b/hello.json
index e36ffac..d387a2f 100644
-------- a/hello.json
+++ b/hello.json
@@ -1,4 +1,5 @@
 [
   "foo": 5,
   "bar": 6,
+ "apple": 7,
 ]
diff --git a/world.json b/world.json
index 14a2818..41f8a01 100644
-------- a/world.json
+++ b/world.json
@@ -1,4 +1,5 @@
 [
   "foo": 5,
- "bar": 6
+ "bar": 6,
+ "apple": 7
 ]

Enter fullscreen mode Exit fullscreen mode

Example blog posts on the topic: https://dontkry.com/posts/code/trailing-commas.html, https://medium.com/@nikgraf/why-you-should-enforce-dangling-commas-for-multiline-statements-d034c98e36f8

Many languages support trailing commas, and some even consider it best practice to use trailing commas.

Ruby

["hello", "world"]
# => ["hello", "world"]
["hello", "world",]
# => ["hello", "world"]

Enter fullscreen mode Exit fullscreen mode

Works the same for hashes.

Python

["hello", "world"]
# Out[1]: ['hello', 'world']
["hello", "world",]
# Out[2]: ['hello', 'world']

Enter fullscreen mode Exit fullscreen mode

Works the same for sets and dictionaries.

Javascript

Mozilla gives a thorough overview of trailing commas in Javascript.

["hello", "world"]
// ['hello', 'world']
["hello", "world",]
// ['hello', 'world']

Enter fullscreen mode Exit fullscreen mode

Probably works for other data types…?

Rust

https://users.rust-lang.org/t/trailing-commas/13993

["hello", "world"]
// vs
["hello", "world",]

Enter fullscreen mode Exit fullscreen mode

Probably works for other data types…?

Julia

https://users.rust-lang.org/t/trailing-commas/13993

( 1, 2 )
# (1, 2)
( 1, 2, )
# (1, 2)

Enter fullscreen mode Exit fullscreen mode

works the same with arrays in Julia.

others

Apparently others do as well: Perl, C#, Swift, etc …

Disagree

Some do not like trailing commas:


R

However, the main dev work I do is in R, which does not support trailing commas.

c("hello", "world")
#> [1] "hello" "world
c("hello", "world", )
#> Error in c("hello", "world", ) : argument 3 is empty

Enter fullscreen mode Exit fullscreen mode

The one caveat is that you will see trailing commas in subsetting procedures of lists, vectors, data.frames, matrices, e.g.,

mtcars[1:3,]

Enter fullscreen mode Exit fullscreen mode

One blogger provides an override to allow trailing commas though I’d imagine it’s not a good idea to use as you probably don’t want such fundamentally different behavior in your own R console compared to others.

I’ve not seen any discussion of trailing commas in R as a language feature, whether good, bad or otherwise. Doesn’t mean it doesn’t exist though :)

Haskell

Like R, doesn’t allow trailing commas!

And in fact, allegedly (I don’t use Haskell):

Because it is much more common to append to lists rather than to prepend, Haskellers have developed the idiom of leading comma:

  ( foo
  , bar
  , baz
  , quux
  )

Enter fullscreen mode Exit fullscreen mode

JSON

Unfortunately for many people JSON does not allow trailing commas

see also: leading with commas

Discussion (6)

Collapse
cristiano profile image
cristiano • Edited

Someone else recently also told me trailing commas help with diffs which is great but I’ve also found that it helped me when I edit an array or object as the comma is already present on the previous entry, it is less prone to errors for me. 😄

Collapse
sckott profile image
Scott Chamberlain Author

Right, good point about being easier to make changes. I also appreciate that.

Collapse
daveparr profile image
Dave Parr

I've seen some people hit the leading comma in R as well. In fact I saw a speaker at Chicago SatRdays live stream using them: twitter.com/just_add_data/status/1...

Collapse
sckott profile image
Scott Chamberlain Author

interesting, is there a link to a talk?

Collapse
daveparr profile image
Dave Parr

Sure, the whole thing is here, and this is the point the leading commas are at

Thread Thread
sckott profile image
Scott Chamberlain Author

thanks!