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

Question

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

Why is the length of arr 2, not 3?

+4

not satan Dec 30 '15 at 1:01

1 answer

Xrxrxr · Accepted Answer · 2014-12-30T01:19:00+0000

This is a feature of array literals introduced in ES3.

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

therefore it [,,,,1,2,3,,]will give an array of length 7 with the first 4, and the last will be empty.

If you put this expression in the console in Firefox 34.0.5, it will say

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

Sources:

section 11.1.4