[removed] Can a comma occur after the last set of values in an array?

Question

I am wondering if a comma trailing an array in javascript is valid?

var settings = {
    \'foo\'  : oof,
    \'bar\' : rab,
};

vs

Answer 1

Most browsers and implementations do allow a trailing comma, the big BUT is the "Internet Explorer".

A trailing comma in most InternetExplorer versions causes BIG trouble. It'll throw wierd, crazy, strange and unreasonable errors .. you have no idea where you're at! This is terrible, you'll fall into deep and serious depressions. The disease also has a name, "evil comma of doom" it was called once.

Conclusion: NEVER.. ever! use a trailing comma in Javascript.

Answer 2

With regards to object literals at least (the question's title mentions array, but the example is an object literal) it depends on where your js is running. Trailing commas in object literals are valid in ES5. Although trailing commas can cause problems on the web (in IE) most minifiers will remove them anyway.

http://www.2ality.com/2011/06/object-literal-comma.html

http://dontkry.com/posts/code/trailing-commas.html

Answer 3

Historically speaking, ES3 did NOT allow a trailing comma when defining an object literal. This was one thing that IE did get right, but most other bowser vendors went against the spec and allowed it anyways. So technically it was a bug in the other browsers that supported it. In ES3, an ObjectLiteral was defined as,

ObjectLiteral
    { }
    { PropertyNameAndValueList }

Later ES5 resolved this issue by going with the majority and legitimizing the trailing comma by putting it in the spec. Now an ObjectLiteral is defined as,

ObjectLiteral
    { }
    { PropertyNameAndValueList } 
    { PropertyNameAndValueList , }

Notice the trailing comma at the end.

Although the trailing comma is defined in an object literal, it is still not allowed in JSON according to ES5. So while the following object literal is valid,

{ foo: "bar", }

the following JSON is not,

'{ "foo": "bar", }'

The grammar for a JSONObject is,

JSONObject
    { }
    { JSONMemberList }

JSONMemberList
    JSONMember  
    JSONMemberList , JSONMember

JSONMember
    JSONString : JSONValue 

In short, if you don't want to worry about spec or browser quirks, then do NOT add a trailing comma.