Javascript Object trailing commas

[shanejann]

As most JS devs might have noticed at one point or another, Javascript allows trailing commas in many instances.

var arr = [
    7,
    8,
    9,    // <-- This is a trailing comma
]

I'd like to get opinions on the topic. I've gotten sent back in Code Review a number of times for this behavior. The arguments against it might be that json doesn't support trailing commas, in which case I would understand why this might be a bad behavior to condone.

[DanielSchaffer]

I like them because:

• It makes it SUPER easy to move lines around in the IDE (in WebStorm it's ALT+UP/ALT+DOWN) - without the trailing comma, you have to manually go fix it
• It makes for cleaner diffs when adding a new line entry - without a trailing comma, you get an extra line just for adding a comma

With a properly configured auto-fixing linter, you don't even need to think about it either.

[shanejann]

My thoughts EXACTLY. I remember being like, "you want em to remove the comma, but look how beautiful that diff is??". It's super nice for anything like arrays, dependency injections, module declarations, etc. (Idk why, but I'm thinking specifically of TypeScript scenarios)

[DanielSchaffer]

Also: I don't think the JSON standard matters in the context of this conversation. Most of the time, we aren't writing JSON, we're writing JavaScript or TypeScript. If you forget and add a hanging comma in JSON, you're likely to be reminded pretty quickly that they're not allowed.