Find all starting indices in a string which are anagrams of a word
This code looks pretty good. I don't see much I would change. This code makes good use of const and let where appropriate.

The only thing that stands out is that splitting a string into an array can be done with the spread syntax instead of calling split().

The first instance:


let sReverse = s.split('').reverse().join('');



Could be changed to 

let sReverse = [...s].reverse().join('');


And the other instance:


const sortAlphabetically = x =&gt; x.toLowerCase().split('').sort().join('')



Could be changed to:

const sortAlphabetically = x =&gt; [...x.toLowerCase()].sort().join('')