From b9249953cdf0acc5ba63478985b4fa52dd5a2556 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Benjamin=20Strau=C3=9F?= <b.strauss@netzbewegung.com>
Date: Tue, 9 Jan 2018 12:48:11 +0100
Subject: [PATCH] feature(functions): cacheAsyncValue

---
 src/functions/functions.js | 44 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 src/functions/functions.js

diff --git a/src/functions/functions.js b/src/functions/functions.js
new file mode 100644
index 0000000..f090b6d
--- /dev/null
+++ b/src/functions/functions.js
@@ -0,0 +1,44 @@
+goog.module('clulib.functions');
+
+/**
+ * Creates an async function that runs the provided parameter 'func'
+ * async function only once.
+ *
+ * When called for the first time, the given async function is called
+ * and its returned result or error is cached. Subsequent calls will
+ * return the cached result or error.
+ *
+ * @param {function():Promise<T>} func
+ * @returns {function():Promise<T>}
+ * @template T
+ */
+function cacheAsyncValue (func) {
+  let done = false;
+  let isError = false;
+
+  /**
+   * @type {T}
+   */
+  let result = null;
+  let error = null;
+
+  return async function () {
+    if (!done) {
+      try {
+        result = await func();
+      } catch (e) {
+        error = e;
+        isError = true;
+      } finally {
+        done = true;
+      }
+    }
+
+    if (isError)
+      throw error;
+    else
+      return result;
+  };
+}
+
+exports = {cacheAsyncValue};