Modern browsers ignore unneeded, trailing commas, so there are no negatives to having them unless you’re supporting an IE 8 application. Since they
make adding new properties simpler, their use is preferred. This rule raises an issue when the last item in a multiline construct (array or object
literal, import or export statement, function declaration or call) does not end with a trailing comma and does not lie on the same line as the closing
curly brace, bracket or parenthesis.
Noncompliant code example
var joe = {
fname: "Joe",
lname: "Smith" // Noncompliant
};
Compliant solution
var joe = {
fname: "Joe",
lname: "Smith", // OK
};
var joe = {
fname: "Joe",
lname: "Smith"}; // OK
