Slice and Splice : wondering why they are different

JavaScript
1

Slice has 2 parameters, start index and end index
Splice has 2 parameters, start index and length (ignoring the third parameter for now)

Seeing as they do almost the same thing, why is there such a difference in the way parameters work?

Why dont both use start index and length notation, or start index and end index notation?

EDIT : they way slice uses the second parameter as index +1 rather than index also does my head in.

To get the second and third element :
using slice(1,2) makes sense. // (index 1, index 2)
using slice(1,3) doesnt makes sense // (index 1, index 3)
using splice(1,2) makes the most sense // (index 1, length 2)

2

They don’t do the same thing, is the basic answer. Slice copies a chunk of an array. Splice removes and/or inserts values from/into an array - you tell it where you want to remove from, how many items to remove, and what values you [optionally] want to insert. They look similar, but splice is for mutating an existing array, whereas slice is for copying bits of an array.

3

The functionality is different, hence the different names.
The sub array indexing should be the same.