Equi.js

// 70%, bad performance
function solution(A) {
    var leftSide
    var rightSide
        
    for (i = 0, arLen = A.length; i < arLen; i++) {
        leftSide = 0;
        rightSide = 0;
        
        for (j = 0; j < i; j++) {
            leftSide += A[j];
            }
            
        for (k = A.length-1; k > i; k--) {
                rightSide += A[k];
            }
            
        if (leftSide === rightSide) {
            return i;
            }
    }return -1
}

// 100%
function solution(A) {        
    var rightSum = A.reduce((acc, val) => acc + val, 0), leftSum = 0;
    
    for (var i = 0; i < A.length; i++) {
        
        rightSum -= A[i];
        
        if (rightSum === leftSum) return i; 
        
        leftSum += A[i];
    }
    
    return -1;
}