Calculate the mod of a collection index

Wrap an index into 0…length−1, also for negative numbers.

JavaScript Version 1.0.0 · MIT · .md

function mod({ index, length }) { return ((index % length) + length) % length; }

Input & expected output

{
  "index": -1,
  "length": 5
}

4

The edge case

{
  "index": 12,
  "length": 5
}

2

Know the limits

• JavaScript’s % keeps the sign of the dividend (-1 % 5 is -1); adding length first fixes it.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Clamp a number to a range — Keep a number inside an inclusive minimum and maximum.
• Round to a number of digits — Apply decimal scaling and JavaScript rounding for display-oriented values.
• Add an ordinal suffix to a number — Append st, nd, rd or th to a positive integer.