Min Avg Two Slice

A non-empty array A consisting of N integers is given. A pair of integers (P, Q), such that 0 ≤ P < Q < N, is called a slice of array A (notice that the slice contains at least two elements). The average of a slice (P, Q) is the sum of A[P] + A[P + 1] + ... + A[Q] divided by the length of the slice. To be precise, the average equals (A[P] + A[P + 1] + ... + A[Q]) / (Q − P + 1).

For example, array A such that:
A[0] = 4
A[1] = 2
A[2] = 2
A[3] = 5
A[4] = 1
A[5] = 5
A[6] = 8
contains the following example slices:
• slice (1, 2), whose average is (2 + 2) / 2 = 2;
• slice (3, 4), whose average is (5 + 1) / 2 = 3;
• slice (1, 4), whose average is (2 + 2 + 5 + 1) / 4 = 2.5.
The goal is to find the starting position of a slice whose average is minimal.

Write a function:
function solution(A); that, given a non-empty array A consisting of N integers, returns the starting position of the slice with the minimal average. If there is more than one slice with a minimal average, you should return the smallest starting position of such a slice.

For example, given array A such that:
A[0] = 4
A[1] = 2
A[2] = 2
A[3] = 5
A[4] = 1
A[5] = 5
A[6] = 8
the function should return 1, as explained above.

Write an efficient algorithm for the following assumptions:
• N is an integer within the range [2..100,000];
• each element of array A is an integer within the range [−10,000..10,000].

hint : check slices in two or three elements

function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    let minStart = 0
    
    let sum = A[0] + A[1] // initial start
    let minAvg = sum / 2
    let avg
    
    for(let i=2; i<A.length; i++){
        sum = sum + A[i] // slice of 3
        avg = sum / 3
        if( minAvg > avg ) {
            minAvg = avg
            minStart = i-2
        }
        
        sum = sum - A[i-2] // slice of 2
        avg = sum / 2
        if( minAvg > avg ){
            minAvg = avg
            minStart = i-1
        }
    }
    return minStart   
}