function binary_search(arr, key) {
    let low = 0,
        high = arr.length - 1
    while (low <= high) {
        let mid = parseInt((high + low) / 2)
        if (key == arr[mid]) {
            return mid
        } else if (key > arr[mid]) {
            low = mid + 1
        } else if (key < arr[mid]) {
            high = mid - 1
        } else {
            return -1
        }
    }
}
let arr = [1, 3, 4, 5, 7, 9],
    key = 7
    
binary_search(arr, key) // 4