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Trailing commas ("final commas"라고도 불립니다)는 새로운 element, parameter 혹은 property들을 JavaScript 코드에 추가할 때 유용합니다. 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.

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 Status Comment
ECMAScript 5.1 (ECMA-262) Standard Added object literal trailing commas.
ECMAScript 2015 (6th Edition, ECMA-262) Standard No change.
ECMAScript Latest Draft (ECMA-262) Draft ES2017 added trailing function commas.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes1 Yes Yes Yes
Trailing comma in object literals Yes Yes19 Yes Yes
Trailing comma in functions58 No52 No No No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes Yes
Trailing comma in object literals Yes Yes Yes4 Yes Yes Yes
Trailing comma in functions No58 No52 No No7.0

See also

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