toposorting a dependency graph

.js

'use strict';

function Node(value, dependencies) {
  dependencies = (dependencies || []);
  this.value = this.valueOf = this.toString = () => value;
  this.dependencies = () => dependencies;
  this.standalone = () => dependencies.length === 0;
  this.equals = node => value === node.value();
}

function flatten(what) {
  if (!Array.isArray(what)) {
    return what;
  } else {
    return what.reduce((one, two) => {
      return one.concat(flatten(two));
    }, []);
  }
}

function indexOf(set, value) {
  // like Array#indexOf(), but for nodes
  let index = -1;
  while ((index += 1) < set.length) {
    if (set[index].equals(value)) {
      return index;
    }
  }
  return -1;
}

function unique(set) {
  return set.filter((value, index) => indexOf(set, value) === index);
}

function resolve(node) {
  if (node.standalone()) {
    return [node];
  } else {
    return unique(flatten([node].concat(
      node.dependencies()
        .map(resolve)
    )));
  }
}

console.log(resolve(
  new Node('a', [
    new Node('b', [
      new Node('c', [
        new Node('d'),
        new Node('e'),
      ])
    ]),
    new Node('c')
  ])
).reverse().join(', '));
// e, d, c, b, a