From f1ae5aad200cf382e1eee617e4e291af9f9c8938 Mon Sep 17 00:00:00 2001
From: Muthu Kumar <muthukumar@thefeathers.in>
Date: Sun, 3 Jun 2018 15:59:51 +0530
Subject: [PATCH] [Promise.object] Resolves all promises in nested, cyclic
 objects

---
 index.js | 117 +++++++++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 77 insertions(+), 40 deletions(-)

diff --git a/index.js b/index.js
index 5b43d01..fb1c187 100644
--- a/index.js
+++ b/index.js
@@ -1,67 +1,104 @@
 "use strict";
 
-const isObject = obj => obj && typeof obj === 'object' && obj.constructor === Object;
+/**
+ * Returns true if x is an object, false otherwise.
+ * @param {any} x
+ * @returns {Boolean}
+ */
+const isObject = x => x &&
+	typeof x === 'object' &&
+	x.constructor === Object;
 
-const makeCircular = (obj, path) => {
-	const start = (o, p, i = 1) => {
-		if (i === path.length) return o[p[0]] = obj;
-		if (p.length > 0) {
-			o[p[0]] = {};
-			return start(o[p[0]], p.slice(1), i + 1);
-		} else return obj;
-	}
-	return start(obj, path);
-};
+/* A well known Symbol. */
+const $SELF = typeof Symbol !== 'undefined' ?
+	Symbol('SELF') :
+	'[~~//-- SELF --//~~]';
 
-const findPath = (obj, query) => {
-	return Object.keys(obj).reduce((acc, key) => {
+/**
+ * Replaces values that match the query parameter
+ * with a reference to the parent parameter.
+ * @param {Object} object Object to make cyclic.
+ * @param {any} query Query to match against.
+ * @returns {Object}
+ */
+const makeCyclic = (object, query) => {
+	const start = obj => Object.keys(obj).reduce((acc, key) => {
 		const value = obj[key];
-		if (value === query) return [...acc, key];
-		if (isObject(value)) return [...acc, ...findPath(value)];
+		if (value === query) {
+			obj[key] = object;
+			return [...acc, key]
+		};
+		if (isObject(value)) return [...acc, ...start(value, query)];
 		else return acc;
 	}, []);
+	return start(object);
 };
 
-// Resolves array
+/**
+ * Promise.map polyfill.
+ * @param {Array.<any>} arr Array of Promises.
+ * @param {Function} functor Function to call resolved values.
+ */
 const PromiseMap = (arr, functor) =>
 	Promise.all(arr.map(x => Promise.resolve(x).then(functor)));
 
-// Resolves objects
+/**
+ * Resolve a flat object's promises.
+ * @param {Object}
+ * @returns {Object}
+ */
 const ResolveObject = obj =>
 	Promise.all(
-		Object.keys(obj).map(key =>
-			Promise.resolve(obj[key]).then(val => obj[key] = val))
+		Object
+		.keys(obj)
+		.map(key =>
+			Promise
+			.resolve(obj[key])
+			.then(val => obj[key] = val))
 	)
 	.then(_ => obj);
 
-// Resolves recrusive deep objects
-const PromiseObject = parent => {
-	const ResolveDeepObject = object =>
+/**
+ * Recursively resolves deep objects with nested promises.
+ * @param {Object} object Object or value to resolve.
+ * @returns {Object} Resolved object.
+ */
+const PromiseObject = object => {
+	let shouldReplaceSelf = false;
+	const ResolveDeepObject = obj =>
 		Promise
-		.resolve(object)
-		.then(obj => {
-			if (Array.isArray(obj)) {
-				return PromiseMap(obj, obj => ResolveDeepObject(obj));
-			} else if (isObject(obj)) {
+		.resolve(obj)
+		.then(resolvedObject => {
+			if (Array.isArray(resolvedObject)) {
+				// Promise and map every item to recursively deep resolve.
+				return PromiseMap(resolvedObject, obj => ResolveDeepObject(obj));
+			} else if (isObject(resolvedObject)) {
 				return ResolveObject(
 					Object
-					.keys(obj)
-					.reduce((acc, key) =>
-						(obj[key] === parent) ? {
+					.keys(resolvedObject)
+					.reduce((acc, key) => {
+						if (resolvedObject[key] === object) {
+							shouldReplaceSelf = true;
+							return {
+								...acc,
+								[key]: $SELF, // Replace with resolved object.
+							}
+						}
+						return {
 							...acc,
-							[key]: '[[ SELF ]]',
-						} : {
-							...acc,
-							[key]: ResolveDeepObject(obj[key]),
-						}, {}));
+							[key]: ResolveDeepObject(resolvedObject[key]),
+						}
+					}, {}));
+
 			}
-			return obj;
+			return resolvedObject;
 		});
-	return ResolveDeepObject(parent)
+	return ResolveDeepObject(object)
 		.then(obj => {
-			const path = findPath(obj, '[[ SELF ]]');
-			return path.length > 0 ? makeCircular(obj, path) : obj;
-		})
+			// Replace $SELF with reference to obj
+			if(shouldReplaceSelf) makeCyclic(obj, $SELF);
+			return obj;
+		});
 };
 
 module.exports = PromiseObject;
\ No newline at end of file