Make `trailingComma` option more granular

This code:

a(b.map(function(c) { return c; }));

is formatted like this:

a(
  b.map(function(c) {
    return c;
  }),
);

with extra comma in a function call.

Demo: https://jlongster.github.io/prettier/#{"content"%3A"a(b.map(function(c) { return c%3B }))%3B%5Cn%22%2C%22options%22%3A%7B%22printWidth%22%3A80%2C%22tabWidth%22%3A2%2C%22singleQuote%22%3Atrue%2C%22trailingComma%22%3Atrue%2C%22bracketSpacing%22%3Atrue%7D%7D

[tabazevedo]

I’d lean towards the --trailing-commas option just not adding them to function calls. The benefit of trailing commas is mostly apparent when adding elements to a list or reordering them and having better git diffs.

Its quite rare that you have a lot of instances in a codebase where there are enough arguments in a function call to warrant that they be on multiple lines and that you’ll change those arguments often enough to reap the benefits of trailing commas (vs. the pain of having to manually edit/ignore files from the formatting).

[joefiorini]

Ran into a good reason not to add trailing commas to function params: the node LTS doesn’t support it! It’s all well and good that babel transpiles my web code es2017, but I’ve purposefully stuck with es2015 in my node code so I don’t have to introduce that complexity.