This is a short post for my future self.
Hi Pablo! Back here already? Yeah, I know, splice, slice, which one is it?? But don't worry, you've got this, only this time we're writting it down!
So, long story short: do you want your array to be mutated? Really? Well, go ahead, sPlice the hell out of it. See what I did there? Capital P, as in "Produces Side Effects, Pal!"
Why, you ask? Because...
The splice() method changes the contents of an array by removing or replacing existing elements and/or adding new elements.
see MDN Docs
Conversly, .slice() method will graciously return the desired data, keeping your beloved original array unmutated. Just tell it where to start and finish (optionally)... slice will comply, no questions asked:
The slice() method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included) where begin and end represent the index of items in that array. The original array will not be modified. see MDN Docs
You can search the webs for examples and more in depth docs, but this should make it easier next time you hit the mental crossroad... splice or slice, you choose (wisely, now)...