Array w/ all elements (except the first) undefined returns wrong length

Question

var arr = ["string", , ]
console.log(arr.length)

Why is the length of arr 2 and not 3?

Trailing commas in array and object literals are ignored, they use to be syntax errors back in the day. — Musa, Dec 30 '14 at 01:04
@Musa Does IE finally permit them? I haven't really worked with it since IE9, when it was still a syntax error. — Michael Berkowski, Dec 30 '14 at 01:05
@Musa should you put your comment as an answer? It seems to answer the question correctly. — Jeremy W, Dec 30 '14 at 01:08
They works in IE11, don't know in which version it started to work. — Musa, Dec 30 '14 at 01:09

score 1 · Accepted Answer · edited May 23 '17 at 11:57

This is a feature of array literals introduced back in ES3.

You can put leading or trilling commas as "empty" elements

so [,,,,1,2,3,,] would yield an array of length 7, with first 4 and the last elements empty.

If you put this expression into the console in Firefox 34.0.5 it would say

Array [ <4 empty slots>, 1, 2, 3, <1 empty slot> ]

Sources:

section 11.1.4

1 Answers1