Trailing commas are those commas found at the end of an array list, object, or function arguments. They can be useful when adding new elements, parameters, or properties to JavaScript code. It just makes it a little more convenient for developers that they can choose to write an array as [1,2,3] or [1,2,3,] (notice the comma in the second example)
JavaScript has allowed trailing commas in arrays and objects for a long time. Finally, in ECMAScript 2017 (ES8), the standard now allows you to add trailing commas to function parameters as well.
That means all the following examples are valid JavaScript code:
Arrays:
var arr = [1, 2, 3,,,];arr.length; // 5arr[3]; // undefinedvar arr2 = [1, 2, 3,];arr2.length; // 3 ...
