Why is [,] a valid array literal in javascript?

Question

For some reason [,] is a valid array literal in javascript. It is a 1-element array. [,,] works as well, and so on. I am looking at the grammar to see why this parses, but as far as I can tell an empty expression shouldn't be allowed there.

Any ideas?

Why wouldn't an empty expression work ? Seems like it should to me — adeneo
– adeneo, Commented Apr 19, 2014 at 21:09
@Johan - [,] would be the same as [undefined] since the last comma with empty item is ignored. — Derek 朕會功夫
– Derek 朕會功夫, Commented Apr 19, 2014 at 21:15
Which grammar are you looking at? You should be looking into the ECMAScript Language Specification, 5.1 Edition (HTML version) — PointedEars
– PointedEars, Commented Apr 19, 2014 at 21:18

William Casarin · Accepted Answer · 2014-04-19 21:34:43Z

2

It seems I was looking at an older language spec. The ECMAScript Language Specification, 5.1 Edition (HTML version)

Defines an ArrayLiteral as:

ArrayLiteral :
    [ Elision opt ]
    [ ElementList ]
    [ ElementList , Elisionopt ]

Elision :
    ,
    Elision ,

The Elision rule seems to define this case

It allows for array element "placeholders" in some sense.

I ran into this issue when dealing with leading commas:

var x = [
  , something
  , somethingElse
  , 2
]

I was wondering why I was getting an undefined for the first element.

Thanks PointedEars and Derek 朕會功夫

edited Apr 19, 2014 at 21:34

answered Apr 19, 2014 at 21:24

William Casarin

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3 Comments

Derek 朕會功夫 Over a year ago

Or in the official PDF specification, page 63, bottom.

PointedEars Over a year ago

You must be mistaken. The Array initialiser was introduced with ECMAScript Edition 3, and the productions ArrayLiteral : [ Elision_opt ] and Elision : , are on page 40 of the December 1999 revision.

RobG Over a year ago

To be clear, an elision doesn't create a member with a value of undefined, it just increases the length, there is no member. var a = ['a',,'b'] creates an array with length 3 and no member at position 1, i.e. '1' in a returns false. And IE has (had?) an issue with trailing elisions, it increases the length by one too many: [,].length should be 1, but IE says 2.

Kai · Accepted Answer · 2014-04-19 21:49:47Z

0

Wow, this is a very good question. I have never noticed it before.

I have made a fiddle that further 'exploits' this phenomena: http://jsfiddle.net/xJnrK/

I think it has something to do with the way javascript handles undefined variables. Javascript uses either 'null' or 'undefined' for empty variables. My guess it is just to prevent code to crash unlike Java where you must give all variables you call a value.

It gets more crazy, undefined is not a reserved word. You can actually override it (in a local scope) using:

function replicateMozilla()
{
    var undefined = '2';

    console.log(undefined);
}

replicateMozilla();

More about undefined: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

edited Apr 19, 2014 at 21:49

answered Apr 19, 2014 at 21:43

Kai

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