const arr = [
  1,
  2,
  3,
];

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

const arr = [1, 2, 3, , ,];
arr.length; // 5

const object = {
  foo: "bar",
  baz: "qwerty",
  age: 42,
};

function f(p) {}
function f(p,) {}

(p) => {};
(p,) => {};

class C {
  one(a,) {}
  two(a, b,) {}
}

const obj = {
  one(a,) {},
  two(a, b,) {},
};

f(p);
f(p,);

Math.max(10, 20);
Math.max(10, 20,);

function f(,) {} // SyntaxError: missing formal parameter
(,) => {};       // SyntaxError: expected expression, got ','
f(,)             // SyntaxError: expected expression, got ','

function f(...p,) {} // SyntaxError: parameter after rest parameter
(...p,) => {}        // SyntaxError: expected closing parenthesis, got ','

// 
[a, b,] = [1, 2];

// 
const o = {
  p: 42,
  q: true,
};
const { p, q, } = o;

const [a, ...b,] = [1, 2, 3];
// SyntaxError: rest element may not have a trailing comma

JSON.parse("[1, 2, 3, 4, ]");
JSON.parse('{"foo" : 1, }');
// SyntaxError JSON.parse: unexpected character
// at line 1 column 14 of the JSON data

JSON.parse("[1, 2, 3, 4 ]");
JSON.parse('{"foo" : 1 }');