why we return 1 and -1 using sort in js code example

Example: why sort() return - 1 js

var arr=[1,2,3,4,5,6,100,999]
arr.sort((a,b)=>{
  if (a%2==b%2) return a-b;
  if (a%2>b%2) return -1;
  return 1;
})
console.log(arr)

//output: [ 1, 3, 5, 999, 2, 4, 6, 100 ]