foldLeft.js

Array.prototype.foldLeft = function(init) {
  function f(sum, list, callback) {
    if (list.length > 0) {
      var head = list.shift();
      return f(callback(sum, head), list, callback);
    }
    return sum;
  }
  var list = this;
  return function(cb) {
    return f(init, list, cb);
  };
};


console.dir(
    [1,1,1].foldLeft(3)((r, x) => r + x)
);