Tail Recursive Tree Recursion

tree-recursion.js

const reduceChildren = ([first, ...rest], accumulator, project, reducer) => (
  rest.length === 0 ?
    recurseTree(first, accumulator, project, reducer) :
    reduceChildren(
      rest,
      recurseTree(first, accumulator, project, reducer),
      project,
      reducer
    )
);

const recurseTree = (tree, accumulator, project, reducer) => (
  tree.length === 1 ?
    reducer(accumulator, project(tree)) :
    reduceChildren(
      tree.slice(1),
      reducer(accumulator, project(tree)),
      project,
      reducer
    )
);

var tree = ['root',
  ['child1', [1], [2], [3]],
  ['child2', [10], [11], ['grandchild2', [22], [23]]],
  ['child3', [30]],
  [100]];

// flatten tree
const identity = (x) => x;
const append = (list, item) => [...list, item];
recurseTree(tree, [], identity, append);

// collect all leaf nodes
const leafValue = ([value, ...rest]) => rest.length === 0 ? [value] : [];
const concat = (list, items) => [...list, ...items];
recurseTree(tree, [], leafValue, concat);

// sum all leaf nodes
const leafValue2 = ([value, ...rest]) => rest.length === 0 ? value : 0;
const sum = (acc, item) => acc + item;
recurseTree(tree, 0, leafValue2, sum);