let array = [1, 3, 5, 8, 10, 13, 15, 18, 20];
let key = 15;
let N = array.length-1;

//Find index number of element in array
function findIndex(arr,key,n){
    for(let i=0;i<n;i++)
        if(arr[i]==key)
            return i;
    return -1;
}   

var result = findIndex(array,key,N);
(result == -1)?console.log('Element not found in array'):
    console.log('Element is at index number ',result);