From 93b35501aa0cdc118aa4552ff70b1a9b04e44600 Mon Sep 17 00:00:00 2001
From: Chia Yu Pai <fantasyatelier@gmail.com>
Date: Wed, 1 Sep 2021 21:06:53 +0800
Subject: [PATCH] add flow-control/promise-waterfall

---
 README.md                         |  9 ++++
 flow-control/promise-waterfall.md | 69 +++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 README.md
 create mode 100644 flow-control/promise-waterfall.md

diff --git a/README.md b/README.md
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+# Rytass Developer Hints
+
+## Type Defination (TypeScript)
+
+## Flow Control
+
+- [Promise Waterfall](flow-control/promise-waterfall.md)
+
+## Data Store/Cache
diff --git a/flow-control/promise-waterfall.md b/flow-control/promise-waterfall.md
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+# Promise Waterfall
+
+## Scenario
+
+當有多個 Promise 需要依序完成，不可同時進行時
+
+## Bad Pattern
+
+### 錯誤的使用 Promise.all
+
+Promise.all 提供的是並行 (Parallels) 執行，所以循序漸進時不可以使用這個 API
+
+```typescript
+const tasks: Promise[];
+
+await Promise.all(tasks);
+```
+
+## Correct Pattern
+
+不使用 Library 的前提下，可以透過 Array.reduce 的方式串起 Promise 並透過 Promise.then 讓其依序執行，並可使用 Promise.catch 在任意錯誤發生時中斷
+
+```typescript
+const tasks: Promise[];
+
+await tasks
+  .map((task) => () => task)
+  .reduce((prev, next) => Promise.resolve());
+```
+
+### Variants A
+
+一包需要循序執行非同步工作的資料
+
+```typescript
+const data = [1,2,3,4,5];
+
+await data
+  .map((element) => async () => {
+    await someAsyncTaskNeedData(element);
+  })
+  .reduce((prev, next) => Promise.resolve());
+```
+
+### Variants B
+
+一包需要循序執行非同步工作的資料，並需要回傳執行結果
+
+> Notice: 需提供回傳 Type，並且因為 reduce 特性，可以同時實作篩選
+
+```typescript
+const data = [1,2,3,4,5];
+
+await data
+  .map((element) => async (results) => {
+    const result = await someAsyncTaskNeedData(element);
+
+    // If filter need
+    // if (result !== condition) {
+    //   return results
+    // }
+
+    return [
+      ...results,
+      result,
+    ];
+  })
+  .reduce((prev, next) => Promise.resolve([] as ResultType[]));
+```