codility.js

function solution(X, A) {
    // write your code in JavaScript (Node.js 4.0.0)
    var l = A.length;
    for(var n = 1; n<l-1;n++){
        sol = A.slice(n),
        sag = A.slice(0,n);
        var x = sol.reduce(function(a,b){if(b==X){a++} return a},0);
        var y = sol.reduce(function(a,b){if(b!=X){a++}return a},0);
        if(x==y){return n-1};
    }
    return null
}