binary_search.js

main()

function main () {
  let a = [5, 7, 15, 28, 29, 31, 39, 58, 68, 70, 95]
  const key = 39
  const index = binarySearch(a, a.length - 1, key)
  console.log('index ', index)
}

function binarySearch (a, n, key) {
  let pl = 0
  let pr = n - 1
  while (pl <= pr) {
    let pc = Math.round((pl + pr) / 2)
    if (a[pc] === key) {
      return pc
    } else if (a[pc] < key) {
      pl = pc + 1
    } else {
      pr = pc - 1
    }
  }
  return -1
}