TL;DR

  • A trailing comma on the final property of a JavaScript object is valid syntax in ECMAScript5
  • ... but it will throw an error in IE8 *surprise* 🎉
  • Trailing commas are also allowed in arrays
  • But if you try to parse an object with a trailing comma to JSON it will throw an error

Do you notice anything different about this object?

{
    cat: "miaow",
    dog: "woof",
    frog: "ribbit",
}

Maybe that trailing comma after “ribbit” strikes you as a bit off. Maybe the neat-freak coder inside you who loves linting wants to backspace the heck out of it 💣 Or maybe you were too busy adding and removing properties from your object to care.

Trailing commas on object properties are valid syntax in ECMAScript 5 (I haven’t checked the spec but I read a StackOverflow question that says it is), and it won’t throw an error. 🚨 Unless, of course, you’re in IE8.

Why?

This style can be considered safer, as it protects you from irritating syntax errors caused by missing commas when you add or remove or rearrange the order of your key-value pairs. It’s also considered neater in commit diffs, as to add a new property to the bottom of an object you only end up modifying one line instead of two.

🚨 But remember:

JSON Object !== JavaScript object, and if you try to parse a JavaScript object with a trailing comma it’ll choke.

JSON.parse('{ "cat": "miaow", "dog": "woof", "frog": "ribbit" }');
// This is valid
JSON.parse('{ "cat": "miaow", "dog": "woof", "frog": "ribbit", }');
// Uncaught SyntaxError: Unexpected token } in JSON

(Don’t forget to double quote your keys too! 😁)

Arrays

Arrays can also handle trailing commas:

['bish','bash','bosh',]     // .length = 3

However don’t be too liberal with those commas in case you accidentally create blank holes in the array where you don’t want them:

['bish','bash','bosh',,]     // .length = 4

Yours sincerely,

So it doesn’t really matter if you’ve got commas all over the place (as long as you bear in mind a couple of caveats), as usual it’s just a question of preference: make sure you decide which style you like and stick to it consistently throughout the codebase. There is a nicely customisable rule in ESLint that will allow you to enforce your chosen preference throughout your project too.