javascript scan function

scan.js

function scan(xs, f, acc) {
  var result = [];

  for (var i = 0; i < xs.length; i++) {
    acc = result[result.push(f(acc, xs[i])) - 1];
  }

  return result;
}

The initial value of the accumulator should be the first element of the prefix scan; here's something similar to the Haskell GHC.List scan (using lodash):

let scanl = (xs, f, ac) =>
  _.concat(ac, xs.length == 0 ? [] : scanl(_.tail(xs), f, f(ac, _.first(xs))));