how are negative array indexes interpreted [duplicate]

Question

I create an array (in Chrome's console)

a = [1, 2, 3]; // [1, 2, 3]

then assign

a[-1] = 123; // 123

this does not throw any error, but the resulting array does not get changed:

a // [1, 2, 3]

but I can read the -1 property successfully:

a[-1] // 123

How does indexing work with Javascript arrays? Why does not it show the new value that I have added? Apparently it treats it like a property. Why?

Here is a link to another stack overflow question that contains answers you might find useful. — user3310334
– user3310334, Commented Mar 10, 2014 at 18:56
Because the standard says so. "A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2**32−1." — user1233508
– user1233508, Commented Mar 10, 2014 at 18:56
The negative arrays are pretty much properties, "Behind the familiar object notation using the dot (i.e., obj.prop) JavaScript is actually accessing the property using association as in obj['prop']." - htmlgoodies.com/beyond/javascript/… Here is a link to another stack overflow question that contains answers you might find useful. — user3310334
– user3310334, Commented Mar 10, 2014 at 19:02

Arjit · Accepted Answer · 2014-03-10 18:59:32Z

As Arrays are objects, It is because you can assign arbitrary properties to an array including negative numbers as well as fractions.

Negative indexes don't actually act like real indexes.

But it will not have any impact on length of array.

1 Answer 1