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.
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 }');
Trailing commas in named imports and named exports
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
Quantifier prefix
//{ DecimalDigits[~Sep], DecimalDigits[~Sep] }
x{n,}
x{n,m}
x{n,m}?
Specifications
Browser compatibility
BCD tables only load in the browser
See also
- Initial ECMAScript proposal: trailing function commas by Jeff Morrison