Find Missing Element

An array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing. Your goal is to find that missing element.

Write a function:
function solution(A);
that, given an array A, returns the value of the missing element.
For example, given array A such that:
A[0] = 2
A[1] = 3
A[2] = 1
A[3] = 5
the function should return 4, as it is the missing element.
Write an efficient algorithm for the following assumptions:
• N is an integer within the range [0..100,000];
• the elements of A are all distinct;
• each element of array A is an integer within the range [1..(N + 1)].

Use trapezoid area (calculated with height=A.length+1) minus existing array's summation to get the missing element.

function missingElement(A) {
    console.log(A)
    var length = A.length; 

    const height = length + 1
    const sum = (1+ height) * height /2 // trapezoid area
    var sumMinusMissing = 0;
    for (i = 0; i < length; i++) { 
        sumMinusMissing += A[i];
    }
    return sum - sumMinusMissing;
}
A1 = [2, 3, 1, 5]
console.log("missingElement : ", missingElement(A1))