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Trailing commas - JavaScript | MDN
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Trailing commas

Baseline Widely available *

This feature is well established and works across many devices and browser versions. Its been available across browsers since 2015 7.

* Some parts of this feature may have varying levels of support.

Trailing commas ("final commas" ) , JavaScript . , trailing comma . .

JavaScript trailing comma , ECMAScript 5 , ECMAScript 2017 .

JSON trailing comma .

In this article

trailing comma

JavaScript trailing comma .

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

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

trailing comma ("") . (sparse) ((dense) ). Array.prototype.forEach() Array.prototype.map() .

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

ECMAScript 5 trailing comma .

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

trailing comma

ECMAScript 2017 trailing comma .

, . Trailing comma length arguments .

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

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

Trailing comma .

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

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

, .

js
f(p);
f(p,);

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

trailing comma

SyntaxError . , rest trailing comma .

js
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 comma

trailing comma .

js
// Trailing comma    
[a, b,] = [1, 2];

// Trailing comma    
var o = {
  p: 42,
  q: true,
};
var {p, q,} = o;

Rest SyntaxError .

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

JSON trailing comma

trailing comma ECMAScript 5 . JSON ES5 JavaScript JSON trailing comma .

SyntaxError .

js
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 trailing comma .

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

Specification
ECMAScript 2027 LanguageSpecification
# prod-Elision
ECMAScript 2027 LanguageSpecification
# prod-ObjectLiteral
ECMAScript 2027 LanguageSpecification
# prod-ArrayLiteral
ECMAScript 2027 LanguageSpecification
# prod-Arguments
ECMAScript 2027 LanguageSpecification
# prod-FormalParameters
ECMAScript 2027 LanguageSpecification
# prod-CoverParenthesizedExpressionAndArrowParameterList
ECMAScript 2027 LanguageSpecification
# prod-NamedImports
ECMAScript 2027 LanguageSpecification
# prod-NamedExports
ECMAScript 2027 LanguageSpecification
# prod-QuantifierPrefix
ECMAScript 2027 LanguageSpecification
# prod-annexB-InvalidBracedQuantifier

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  11. Classes
    1. constructor
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    5. static
    6. Class static initialization blocks
    1. Backreference: \1, \2
    2. Capturing group: (...)
    3. Character class escape: \d, \D, \w, \W, \s, \S
    4. Character class: [...], [^...]
    5. Character escape: \n, \u{...}
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    11. Modifier: (?ims-ims:...)
    12. Named backreference: \k<name>
    13. Named capturing group: (?<name>...)
    14. Non-capturing group: (?:...)
    15. Quantifier: *, +, ?, {n}, {n,}, {n,m}
    16. Unicode character class escape: \p{...}, \P{...}
    17. Wildcard: .
    18. Word boundary assertion: \b, \B
    1. AggregateError: No Promise in Promise.any was resolved
    2. Error: Permission denied to access property "x"
    3. InternalError: too much recursion
    4. RangeError: argument is not a valid code point
    5. RangeError: BigInt division by zero
    6. RangeError: BigInt negative exponent
    7. RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'
    8. RangeError: invalid array length
    9. RangeError: invalid date
    10. RangeError: precision is out of range
    11. RangeError: radix must be an integer
    12. RangeError: repeat count must be less than infinity
    13. RangeError: repeat count must be non-negative
    14. RangeError: x can't be converted to BigInt because it isn't an integer
    15. ReferenceError: "x" is not defined
    16. ReferenceError: assignment to undeclared variable "x"
    17. ReferenceError: can't access lexical declaration`X' before initialization
    18. ReferenceError: must call super constructor before using 'this' in derived class constructor
    19. ReferenceError: super() called twice in derived class constructor
    20. SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code
    21. SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
    22. SyntaxError: "use strict" not allowed in function with non-simple parameters
    23. SyntaxError: "x" is a reserved identifier
    24. SyntaxError: \ at end of pattern
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    27. SyntaxError: arguments is not valid in fields
    28. SyntaxError: await is only valid in async functions, async generators and modules
    29. SyntaxError: await/yield expression can't be used in parameter
    30. SyntaxError: cannot use ?? unparenthesized within || and && expressions
    31. SyntaxError: character class escape cannot be used in class range in regular expression
    32. SyntaxError: continue must be inside loop
    33. SyntaxError: duplicate capture group name in regular expression
    34. SyntaxError: duplicate formal argument x
    35. SyntaxError: for-in loop head declarations may not have initializers
    36. SyntaxError: function statement requires a name
    37. SyntaxError: functions cannot be labelled
    38. SyntaxError: getter and setter for private name #x should either be both static or non-static
    39. SyntaxError: getter functions must have no arguments
    40. SyntaxError: identifier starts immediately after numeric literal
    41. SyntaxError: illegal character
    42. SyntaxError: import declarations may only appear at top level of a module
    43. SyntaxError: incomplete quantifier in regular expression
    44. ReferenceError: invalid assignment left-hand side
    45. SyntaxError: invalid BigInt syntax
    46. SyntaxError: invalid capture group name in regular expression
    47. SyntaxError: invalid character in class in regular expression
    48. SyntaxError: invalid class set operation in regular expression
    49. SyntaxError: invalid decimal escape in regular expression
    50. SyntaxError: invalid identity escape in regular expression
    51. SyntaxError: invalid named capture reference in regular expression
    52. SyntaxError: invalid property name in regular expression
    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
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    56. SyntaxError: invalid unicode escape in regular expression
    57. SyntaxError: JSON.parse: bad parsing
    58. SyntaxError: label not found
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    60. SyntaxError: missing ) after argument list
    61. SyntaxError: missing ) after condition
    62. SyntaxError: missing ] after element list
    63. SyntaxError: missing } after function body
    64. SyntaxError: missing } after property list
    65. SyntaxError: missing = in const declaration
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    71. SyntaxError: nothing to repeat
    72. SyntaxError: numbers out of order in {} quantifier.
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    77. SyntaxError: raw bracket is not allowed in regular expression with unicode flag
    78. SyntaxError: redeclaration of formal parameter "x"
    79. SyntaxError: reference to undeclared private field or method #x
    80. SyntaxError: rest parameter may not have a default
    81. SyntaxError: return not in function
    82. SyntaxError: setter functions must have one argument
    83. SyntaxError: string literal contains an unescaped line break
    84. SyntaxError: super() is only valid in derived class constructors
    85. SyntaxError: tagged template cannot be used with optional chain
    86. SyntaxError: Unexpected '#' used outside of class body
    87. SyntaxError: Unexpected token
    88. SyntaxError: unlabeled break must be inside loop or switch
    89. SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'
    90. SyntaxError: use of super property/member accesses only valid within methods or eval code within methods
    91. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    92. ReferenceError: deprecated caller or arguments usage
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    99. TypeError: already executing generator
    100. TypeError: BigInt value can't be serialized in JSON
    101. TypeError: calling a builtin X constructor without new is forbidden
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    103. TypeError: can't assign to property "x" on "y": not an object
    104. TypeError: can't convert BigInt to number
    105. TypeError: can't convert x to BigInt
    106. TypeError: can't define property "x": "obj" is not extensible
    107. TypeError: can't delete non-configurable array element
    108. TypeError: can't redefine non-configurable property "x"
    109. TypeError: can't set prototype of this object
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    111. TypeError: cannot use 'in' operator to search for 'x' in 'y'
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    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
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    118. TypeError: invalid Array.prototype.sort argument
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    123. TypeError: "x" has no properties
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    128. TypeError: X.prototype.y called on incompatible type
    129. URIError: malformed URI sequence
    130. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    131. Warning: unreachable code after return statement
    1. JavaScript
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    3. Lexical
    4. Iteration
    7. Trailing commas
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