Tape-Equilibrium Codility Training

Your solution is already O(N). You need to remove the abs from sumleft and sumright.

if (Math.abs( sumleft - sumright ) < ans)
{
  ans = Math.abs( sumleft - sumright );
}

Also before the second for loop,

ans =Math.abs( sumleft - sumright );

It should work.

100%, in Javascript

var i, ll = A.length, tot = 0, upto = 0, min = Number.MAX_INT;

for (i=0; i<ll; i++) tot += A[i];

for (i=0; i<ll-1; i++)
{
    upto += A[i];
    var a1 = upto, a2 = tot - a1, dif = Math.abs(a1 - a2);
    if (dif < min)
         min = dif;
}

return min;