Why does the JavaScript array operation [1,2,3,4][1,2] return 3?

Question

I am not able to understand why this JavaScript array is returning 3. What is the logic behind returning 3?

I just executed this in a developer's tool:

[1,2,3,4][1,2]

`[1,2,3,4]` is an array. In `[1,2]` `1,2` Comma operator causes the result as `2`. and `arr[2]` returns element at second index - **3**. — Tushar, Feb 14 '16 at 15:53
@PreferenceBean by which keyword you searched it? i searched a lot but didnt came across that answer — brk, Feb 14 '16 at 16:28
@user2181397: I remembered it so only had to go into my profile, Answers section, sort by votes, and copy/paste the URL to the top one. — Lightness Races in Orbit, Feb 14 '16 at 16:38

Tushar · Accepted Answer · 2016-02-14T16:03:10.970

4

Breakdown of [1,2,3,4][1,2]

[1,2,3,4]: A normal array of four elements
1,2: Comma operator returning the last operand. Thus, giving result as 2
arr[2]: Accessing the element from array using Bracket Notation/Property Accessor.

Thus the resulting equivalent statement will be [1, 2, 3, 4][2].

This is equivalent to

var index = 1, 2; // Note the comma operator. This is same as `var index = 2;`
var arr = [1, 2, 3, 4];

arr[index]; // arr[2] = 3

edited Feb 14 '16 at 16:03

answered Feb 14 '16 at 16:00

Tushar

That's so weird. I never knew the comma operator did that. Thanks! – sg.cc Feb 14 '16 at 16:03

1 Answers1