January 16, 2018
2 min read
Slice can take in two parameters for the beginning and end index: arr.slice([begin[, end]]). The array.slice() function has some interesting logic when either of those parameters is a negative value. Positive index values start iterating from the beginning of the array while negative values iterate from the end of the array.
msdn documentation for the slice function explains this:
A negative index can be used, indicating an offset from the end of the sequence. slice(-2) extracts the last two elements in the sequence.
A negative index can be used, indicating an offset from the end of the sequence. slice(2,-1) extracts the third element through the second-to-last element in the sequence.
This caused me some confusing issues when using the array.findIndex() function did not find the object I was looking for. It returns a value of -1 and caused me to start slicing with negative indexes!
const arr = [1, 2, 3];arr.slice(-2);[2, 3]
const arr = [1, 2, 3];arr.slice(1, -1);[2]