functors-e10.js

function treeNode(value, left, right) {
    return {
        value: value,
        left: left,
        right: right,
        map: function(transformation) {
            var nextValue = transformation(nextValue);

            var nextLeft;
            if (left) nextLeft = left.map(transformation);
          
            var nextRight;
            if (right) nextRight = right.map(transformation);

            return treeNode(nextValue, nextLeft, nextRight);
        }
    };
}

You've got a typo: line 7 should read:

var nextValue = transformation(value);

As it currently stands, it's calling transformation on a value which doesn't yet exist. (It only runs without complaint because of the use of var to bind the variable name.)