const minMax = (A) => A.reduce((a, b) => {
  if (!Array.isArray(a)) a = [a, a]
  a[0] = a[0] > b ? b : a[0]
  a[1] = a[1] < b ? b : a[1]
  return a
})

// test
a = [10, 5, 6, 12, 30, 10]
console.log(minMax(a)) // [ 5, 30 ]

// extra
const min = (A) => A.reduce((a, b) => a < b ? a : b)
const max = (A) => A.reduce((a, b) => a > b ? a : b)