trailing comma javascript

Trailing commas (sometimes called "final commas") can be useful when adding
new elements, parameters, or properties to JavaScript code.
If you want to add a new property, you can simply add a new line without
modifying the previously last line if that line already uses a trailing comma.
This makes version-control diffs cleaner and editing code might be less
troublesome.

//For arrays
var arr = [
  1, 
  2, 
  3, 
];

arr; // [1, 2, 3]
arr.length; // 3

//For objects
var object = { 
  foo: "bar", 
  baz: "qwerty",
  age: 42,
};