Always Add That Trailing Comma!

Let's say you are defining a long array in PHP. It's so long that you put every element on its own line like so:

$schema = [
    'userID:i' => 'ID of the user.',
    'name:s' => 'Name of the user.',
    'password:s' => 'Password of the user.',
];

Notice how the last item in the array has a comma after it? That's not a mistake and has been supported in PHP for a long time. We haven't used this feature all that much, but it is considered a best practice. Why? Here are a couple of reasons:

1. If you decide to move lines around you can do so without having to add or remove commas depending on whether the line is the last item in the array.
2. If you add lines to the array in a later commit then the diffs are cleaner because only the line you added will show as changed.

It's really number 2 that we want. Clean diffs make pull requests easier to read for humans and easier to merge for computers. Trailing commas are one thing humans and computers can agree on.

Here are a few gotchas and tips when using trailing commas:

1. Don't use trailing commas for arrays that are on a single line. That's gross.
2. Don't go back and change every array declaration you can find. That's a make-work project we don't need to do. Just try and use trailing commas on new arrays and maybe old arrays that you touch doing some other task.
3. You can put trailing commas in typescript as well. In fact, Adam has put a lint rule to enforce this.
4. Typescript also allows trailing commas in function declarations and calls; PHP does not.