Did you know the following JS Trailing Comma Facts ?

• In Arrays, trailing commas are ignored. More than 1 comma if used, produces an elision or hole

var array = [10,20,30,];
console.log(array); //[10,20,30]
console.log(array.length) //3

But if,

var array = [10,20,30,,,,];
console.log(array); //[10,20,30,,,] -> empty * 3
console.log(array.length) //6

An array with holes is called as sparse array. When iterating array (using forEach() or map()), holes are skipped

• Function / Method parameter definitions or invocation, trailing commas are legal

function funWithJS(arg) {}
function funWithJS(arg,) {}

Or,

(arg) => {}
(arg,) => {}

are valid function calls.

Similarly,

class Shape{
circleArea(radius,) {return 3.14 * radius}
rectangleArea(length,width,){return length * width}
} var obj = new Shape();

console.log(obj.circleArea(5)); // 15.700000000000001
console.log(obj.rectangleArea(5,10)); //50

When invoked with trailing comma, also yields the same result.

console.log(obj.circleArea(5,)); // 15.700000000000001
console.log(obj.rectangleArea(5,10,)); //50

Note: - Function or Method invocation or definition containing only a comma wil throw a SyntaxError.

• Trailing commas in Destructuring assignment* or object literals is legal

var user = {
name: "WebAtoms",
type: "Framework",
};

A trailing comma is allowed on the left-hand side when using destructuring assignment.

// array destructuring with trailing comma
[i, j,] = [10, 20];

// object destructuring with trailing comma
var user = {
  id: 100,
  active: true,
};
var {id, active,} = user;

Note: - Trailing comma when used with a rest element, will give a SyntaxError. Also, JSON doesn't allow trailing commas.

var [a,...b,] =  [1,2,3]; //SyntaxError

Happy Learning!