code: function dedupeNonExistent(nonExistent) {
    const deduped = new Set(nonExistent);
    nonExistent.length = deduped.size;
  
    let i = 0;
    for (const elem of deduped) {
      nonExistent[i] = elem;
      i++;
    }
  }
docs: Mutate the list input to do a dereferenced modification of the user-supplied list