SOURCE

var arr = [2,5,6,7,8,9,12,23,34,55,67,89,209]

function seaching(array,target){
    let start = 0,end = array.length - 1,middle,element
    while(start <= end){
        middle = Math.floor((start + end) / 2)
        element = array[middle]
        if(target === element){
            return middle
        }else if(target < element){
            end = middle - 1
        }else{
            start = middle + 1
        }
    }
    return -1
}  

console.log(seaching(arr,12))