Sorting with priority

priority-sorting.js

const prioritizedVals = {
  a: 0,
  b: 1,
  c: 2
}

function sortWithPriority(a, b) {
  // if it's not a prioritized value the sort value is Infinity and falls back to default sorting
  const s1 = prioritizedVals[a] || Infinity
  const s2 = prioritizedVals[b] || Infinity
  // s1 - s2 will return NaN when both s1 and s2 are Infinity which is falsy, and falls back to the default sorting
  // s1 and s2 will be a negative number (-Infinity) if s2 is not a prioritized value which puts a before b
  // s1 and s2 will be a positive number (Infinity) if s1 is not a prioritized value which puts a after b
  // s1 and s2 will do proper math when they are both prioritized values
  return s1 - s2 || a < b ? -1 : a > b ? 1 : 0
}