Disallow or Enforce Dangling Commas (comma-dangle)

Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec. However, IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.

var foo = {
    bar: "baz",
    qux: "quux",
};

On the other hand, trailing commas simplify adding and removing items to objects and arrays, since only the lines you are modifying must be touched.

Rule Details

This rule enforces consistent use of trailing commas in object and array literals.

This rule takes one argument, which can be one of the following options:

"always" - warn whenever a missing comma is detected.
"always-multiline" - warn if there is a missing trailing comma on arrays or objects that span multiple lines, and warns if there is a trailing comma present on single line arrays or objects.
"only-multiline" - warn whenever a trailing comma is detected on single line nodes.
"never" - warn whenever a trailing comma is detected.

The default value of this option is "never".

The following patterns are considered problems when configured "never":

/*eslint comma-dangle: [2, "never"]*/

var foo = {
    bar: "baz",
    qux: "quux",   /*error Unexpected trailing comma.*/
};

var arr = [1,2,];  /*error Unexpected trailing comma.*/

foo({
  bar: "baz",
  qux: "quux",     /*error Unexpected trailing comma.*/
});

The following patterns are not considered problems when configured "never":

/*eslint comma-dangle: [2, "never"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

The following patterns are considered problems when configured "always":

/*eslint comma-dangle: [2, "always"]*/

var foo = {
    bar: "baz",
    qux: "quux"   /*error Missing trailing comma.*/
};

var arr = [1,2];  /*error Missing trailing comma.*/

foo({
  bar: "baz",
  qux: "quux"     /*error Missing trailing comma.*/
});

The following patterns are not considered problems when configured "always":

/*eslint comma-dangle: [2, "always"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

The following patterns are considered problems when configured "always-multiline":

/*eslint comma-dangle: [1, "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux"                         /*error Missing trailing comma.*/
};

var foo = { bar: "baz", qux: "quux", }; /*error Unexpected trailing comma.*/

var arr = [1,2,];                       /*error Unexpected trailing comma.*/

var arr = [1,
    2,];                                /*error Unexpected trailing comma.*/

var arr = [
    1,
    2                                   /*error Missing trailing comma.*/
];

foo({
  bar: "baz",
  qux: "quux"                           /*error Missing trailing comma.*/
});

The following patterns are not considered problems when configured "always-multiline":

/*eslint comma-dangle: [2, "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

foo({
  bar: "baz",
  qux: "quux",
});

The following patterns are considered problems when configured "only-multiline":

/*eslint comma-dangle: [1, "only-multiline"]*/

var foo = { bar: "baz", qux: "quux", }; /*error Unexpected trailing comma.*/

var arr = [1,2,];                       /*error Unexpected trailing comma.*/

var arr = [1,
    2,];                                /*error Unexpected trailing comma.*/

The following patterns are not considered problems when configured "only-multiline":

/*eslint comma-dangle: [2, "only-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux",
});

foo({
  bar: "baz",
  qux: "quux"
});

Disallow or Enforce Dangling Commas (comma-dangle)

Rule Details

When Not To Use It

Version

Resources