Trailing commas

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 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.

JavaScript has allowed trailing commas in array literals since the beginning, and later added them to object literals, and more recently, to function parameters and to named imports and named exports.

JSON, however, disallows trailing commas.

Arrays

JavaScript ignores trailing commas in arrays:

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

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

If more than one trailing comma is used, an elision (or hole) is produced. An array with holes is called sparse (a dense array has no holes). When iterating arrays for example with Array.prototype.forEach() or Array.prototype.map(), array holes are skipped.

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

Objects

Starting with ECMAScript 5, trailing commas in object literals are legal as well:

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

ECMAScript 2017 allows trailing commas in function parameter lists.

Parameter definitions

The following function definition pairs are legal and equivalent to each other. Trailing commas don't affect the length property of function declarations or their arguments object.

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

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

The trailing comma also works with method definitions for classes or objects:

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

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

Function calls

The following function invocation pairs are legal and equivalent to each other.

f(p);
f(p,);

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

Illegal trailing commas

Function parameter definitions or function invocations only containing a comma will throw a SyntaxError. Furthermore, when using a rest parameters, trailing commas are not allowed:

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 trailing comma is also allowed on the left-hand side when using destructuring assignment:

// array destructuring with trailing comma
[a, b,] = [1, 2];

// object destructuring with trailing comma
var o = {
  p: 42,
  q: true,
};
var {p, q,} = o;

Again, when using a rest element, a SyntaxError will be thrown:

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

Trailing commas in objects were only introduced in ECMAScript 5. As JSON is based on JavaScript's syntax prior to ES5, trailing commas are not allowed in JSON.

Both lines will throw a SyntaxError:

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

Omit the trailing commas to parse the JSON correctly:

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

Trailing commas are valid in named imports and named exports.

named imports

  import {
    A,
    B,
    C,
  } from 'D'

  import { X, Y, Z } from 'W'

  import { A as B, C as D, E as F } from 'Z'; //Renaming imports

named exports

  export {
    A,
    B,
    C
  }

  export { A, B, C };

  export { A as B, C as D, E as F }; // Renaming exports

• Initial ECMAScript proposal: trailing function commas by Jeff Morrison