.js

var node = (left, v, right) => ({
  left: left,
  v: v,
  right: right,
  map: f => node(left.map(f), f(v), right.map(f)), 
});

var leaf = v => ({  v: v,  map: f => leaf(f(v)), });

var tree = node(leaf(1),3, node(leaf(5), 7, leaf(13)));