Web/JavaScript/Reference/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 simply 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 (ECMAScript 5) and most recently (ECMAScript 2017) to function parameters.

JSON, however, disallows trailing commas.

Syntax

Examples

Trailing commas in literals

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,
};

Trailing commas in functions

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 ','

Trailing commas in destructuring

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 JSON

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 }');

Specifications

Specification

ECMAScript (ECMA-262)

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
Trailing commas	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 9	Opera Full support 9.5	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100
Trailing comma in functions	Chrome Full support 58	Edge Full support 14	Firefox Full support 52	IE No support No	Opera Full support 45	Safari Full support 10	WebView Android No support No	Chrome Android Full support 58	Firefox Android Full support 52	Opera Android Full support 43	Safari iOS Full support 10	Samsung Internet Android Full support 7.0	nodejs Full support 8.0.0
Trailing comma in object literals	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 9	Opera Full support 9.5	Safari Full support 3	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100

Legend

Full support: Full support
No support: No support