Move the rejected promises part of no-throw to new rule no-reject

README.md

@@ -52,6 +52,7 @@ In addition to immutable rules this project also contains a few rules for enforc
   * [no-loop-statement](#no-loop-statement)
   * [no-throw](#no-throw)
   * [no-try](#no-try)
+  * [no-reject](#no-reject)
 * [Recommended built-in rules](#recommended-built-in-rules)
 
 ## Immutability rules
@@ -451,7 +452,25 @@ function divide(x: number, y: number): number | Error {
 }
 ```
 
-And likewise for async functions:
+Or in the case of an async function, a rejected promise should be returned.
+
+```typescript
+async function divide(x: Promise<number>, y: Promise<number>): Promise<number> {
+  const [xv, yv] = await Promise.all([x, y]);
+
+  return yv === 0
+    ? Promise.reject(new Error("Cannot divide by zero."))
+    : xv / yv;
+}
+```
+
+### no-try
+
+Try statements are not part of functional programming. See [no-throw](#no-throw) for more information.
+
+### no-reject
+
+You can view a `Promise` as a result object with built-in error (something like `{ value: number } | { error: Error }`) in which case a rejected `Promise` can be viewed a returned result and thus fits with functional programming. You can also view a rejected promise as something similar to an exception and as such something that does not fit with functional programming. If your view is the latter you can use the `no-reject` rule to disallow rejected promises.
 
 ```typescript
 async function divide(
@@ -460,14 +479,16 @@ async function divide(
 ): Promise<number | Error> {
   const [xv, yv] = await Promise.all([x, y]);
 
+  // Rejecting the promise is not allowed so resolve to an Error instead
+
+  // return yv === 0
+  //   ? Promise.reject(new Error("Cannot divide by zero."))
+  //   : xv / yv;
+
   return yv === 0 ? new Error("Cannot divide by zero.") : xv / yv;
 }
 ```
 
-### no-try
-
-Try statements are not part of functional programming. See [no-throw](#no-throw) for more information.
-
 ## Options
 
 ### Using the `ignore-local` option

src/noRejectRule.ts

@@ -0,0 +1,30 @@
+import * as ts from "typescript";
+import * as Lint from "tslint";
+import {
+  createInvalidNode,
+  CheckNodeResult,
+  createCheckNodeRule
+} from "./shared/check-node";
+
+type Options = {};
+
+// tslint:disable-next-line:variable-name
+export const Rule = createCheckNodeRule(
+  checkNode,
+  "Unexpected reject, return an error instead."
+);
+
+function checkNode(
+  node: ts.Node,
+  _ctx: Lint.WalkContext<Options>
+): CheckNodeResult {
+  if (
+    ts.isPropertyAccessExpression(node) &&
+    ts.isIdentifier(node.expression) &&
+    node.expression.text === "Promise" &&
+    node.name.text === "reject"
+  ) {
+    return { invalidNodes: [createInvalidNode(node, [])] };
+  }
+  return { invalidNodes: [] };
+}

src/noThrowRule.ts

@@ -19,16 +19,7 @@ function checkNode(
   node: ts.Node,
   _ctx: Lint.WalkContext<Options>
 ): CheckNodeResult {
-  if (utils.isThrowStatement(node)) {
-    return { invalidNodes: [createInvalidNode(node, [])] };
-  }
-  if (
-    ts.isPropertyAccessExpression(node) &&
-    ts.isIdentifier(node.expression) &&
-    node.expression.text === "Promise" &&
-    node.name.text === "reject"
-  ) {
-    return { invalidNodes: [createInvalidNode(node, [])] };
-  }
-  return { invalidNodes: [] };
+  return utils.isThrowStatement(node)
+    ? { invalidNodes: [createInvalidNode(node, [])] }
+    : { invalidNodes: [] };
 }

test/rules/no-reject/default/test.ts.lint

@@ -0,0 +1,18 @@
+const error = new Error();
+
+function foo(): Promise<number> {
+    if (Math.random() > 0.5) {
+        return Promise.reject(new Error("bar"))
+               ~~~~~~~~~~~~~~ [failure]
+    }
+    return Promise.resolve(10)
+}
+
+function bar(): Promise<number | Error> {
+    if (Math.random() > 0.5) {
+        return Promise.resolve(new Error("foo"))
+    }
+    return Promise.resolve(10)
+}
+
+[failure]: Unexpected reject, return an error instead.

test/rules/no-reject/default/tslint.json

@@ -0,0 +1,6 @@
+{
+  "rulesDirectory": ["../../../../rules"],
+  "rules": {
+    "no-reject": true
+  }
+}

test/rules/no-throw/default/test.ts.lint

@@ -9,19 +9,4 @@ throw error;
 throw new Error();
 ~~~~~~~~~~~~~~~~~~ [failure]
 
-function foo(): Promise<number> {
-    if (Math.random() > 0.5) {
-        return Promise.reject(new Error("bar"))
-               ~~~~~~~~~~~~~~ [failure]
-    }
-    return Promise.resolve(10)
-}
-
-function bar(): Promise<number | Error> {
-    if (Math.random() > 0.5) {
-        return Promise.resolve(new Error("foo"))
-    }
-    return Promise.resolve(10)
-}
-
 [failure]: Unexpected throw, throwing exceptions is not functional.