Promise.tree

promise-tree.js

/**
 * A Tree is an Object that may contain other Trees, Objects, Promises,
 * or primitives. By definition, it contains no cycles.
 * @typedef [Object] Tree
 */

/**
 * @function tree
 * @memberof Promise
 * @param [Tree] tree
 * @returns [Promise] - A Tree with all promises resolved.
 *
 * @description
 * Works like $q.all, but for arbitrarily nested objects.
 *
 * @example
 * ```javascript
 *  const tree = {
 *    one: Promise.resolve(1),
 *    two: {
 *      three: 'three',
 *      four: Promise.resolve(4),
 *    },
 *    five: 5,
 *  };
 *
 *  Promise.tree(tree).then((result) => {
 *    assert.deepEqual(result, {
 *      one: 1,
 *      two: {
 *        three: 'three',
 *        four: 4,
 *      },
 *      five: 5,
 *    });
 *  });
 * ```
 */

const isObject = (x) => Object(x) === x;
const isTree = (x) => isObject(x) && ! (typeof x.then === 'function');
const transform = (x) => isTree(x) ? Promise.tree(x) : x;
const keys = (x) => Object.keys(x);
const objectKeyReduce = (fn, init, x) => keys(x).reduce(fn, init);
const merge = (...args) => Object.assign({}, ...args);

function zip(names) {
  return (values) => names.reduce(
    (map, name, index) => merge(map, {
      [name]: values[index],
    }), {}
  );
}

Promise.tree = (tree) => (
  Promise.all(
    objectKeyReduce(
      (promises, key) => (
        [...promises, transform(tree[key])]
      ), [], tree
    )
  ).then(
    zip(keys(tree))
  )
);