SOURCE

function bsearch(arr,key){
  var low = 0;
  var high = arr.length-1;
  var mid = Math.floor((low+high)/2);
  while (low<=high){
    if(arr[mid]==key){
      return mid;
    }else if(arr[mid]>key){
      high = mid -1;
    }else if (arr[mid]<key){
      low = mid +1;
    }
  }
  return -1;
}
arr = [1,2,3,4,6,8,9];
key = 8;
console.log(bsearch(arr,key))