New rule no-return-await

src/configs/all.ts

@@ -116,6 +116,7 @@ export const rules = {
     "no-misused-new": true,
     "no-null-keyword": true,
     "no-object-literal-type-assertion": true,
+    "no-return-await": true,
     "no-shadowed-variable": true,
     "no-string-literal": true,
     "no-string-throw": true,

src/configs/latest.ts

@@ -65,11 +65,12 @@ export const rules = {
     },
 
     // added in v5.8
-    "no-duplicate-switch-case": true,
+    "ban-comma-operator": true,
     "jsdoc-format": {
         options: "check-multiline-start",
     },
-    "ban-comma-operator": true,
+    "no-duplicate-switch-case": true,
+    "no-return-await": true,
 };
 // tslint:enable object-literal-sort-keys
 

src/rules/noReturnAwaitRule.ts

@@ -0,0 +1,116 @@
+/**
+ * @license
+ * Copyright 2017 Palantir Technologies, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { isFunctionScopeBoundary, isTryStatement } from "tsutils";
+import * as ts from "typescript";
+
+import * as Lint from "../index";
+
+export class Rule extends Lint.Rules.AbstractRule {
+    /* tslint:disable:object-literal-sort-keys */
+    public static metadata: Lint.IRuleMetadata = {
+        ruleName: "no-return-await",
+        description: "Disallows unnecessary `return await`.",
+        rationale: Lint.Utils.dedent`
+            An async function always wraps the return value in a Promise.
+            Using \`return await\` just adds extra time before the overreaching promise is resolved without changing the semantics.
+        `,
+        optionsDescription: "Not configurable.",
+        options: null,
+        optionExamples: [true],
+        type: "functionality",
+        typescriptOnly: false,
+        hasFix: true,
+    };
+    /* tslint:enable:object-literal-sort-keys */
+
+    public static FAILURE_STRING = "Unnecessary 'await'.";
+
+    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+        return this.applyWithFunction(sourceFile, walk);
+    }
+}
+
+function walk(ctx: Lint.WalkContext<void>) {
+    return ts.forEachChild(ctx.sourceFile, function cb(node): void {
+        if (node.kind === ts.SyntaxKind.AwaitExpression && isUnnecessaryAwait(node)) {
+            const {expression} = node as ts.AwaitExpression;
+            const keywordStart = expression.pos - "await".length;
+            ctx.addFailure(
+                keywordStart,
+                expression.pos,
+                Rule.FAILURE_STRING,
+                Lint.Replacement.deleteFromTo(keywordStart, expression.getStart(ctx.sourceFile)),
+            );
+        }
+        return ts.forEachChild(node, cb);
+    });
+}
+
+function isUnnecessaryAwait(node: ts.Node): boolean {
+    while (true) {
+        const parent = node.parent!;
+        outer: switch (parent.kind) {
+            case ts.SyntaxKind.ArrowFunction:
+                return true;
+            case ts.SyntaxKind.ReturnStatement:
+                return !isInsideTryBlock(parent.parent!);
+            case ts.SyntaxKind.ParenthesizedExpression:
+                break;
+            case ts.SyntaxKind.ConditionalExpression:
+                if ((parent as ts.ConditionalExpression).condition === node) {
+                    return false;
+                }
+                break;
+            case ts.SyntaxKind.BinaryExpression:
+                if ((parent as ts.BinaryExpression).right === node) {
+                    switch ((parent as ts.BinaryExpression).operatorToken.kind) {
+                        case ts.SyntaxKind.AmpersandAmpersandToken:
+                        case ts.SyntaxKind.BarBarToken:
+                        case ts.SyntaxKind.CommaToken:
+                            break outer;
+                    }
+                }
+                return false;
+            default:
+                return false;
+        }
+        node = parent;
+    }
+}
+
+function isInsideTryBlock(node: ts.Node): boolean {
+    while (node.parent !== undefined) {
+        if (isFunctionScopeBoundary(node)) {
+            return false;
+        }
+        if (isTryStatement(node.parent)) {
+            if (
+                // statements inside the try block always have an error handler, either catch or finally
+                node.parent.tryBlock === node ||
+                // statement inside the catch block only have an error handler if there is a finally block
+                node.parent.finallyBlock !== undefined && node.parent.catchClause === node
+            ) {
+                return true;
+            }
+            node = node.parent.parent!;
+        } else {
+            node = node.parent;
+        }
+    }
+    return false;
+}

test/rules/no-return-await/test.ts.fix

@@ -0,0 +1,80 @@
+async function foo() {
+    return x;
+}
+(async () => x);
+
+async function allowTypeAssertion() {
+    return (await x)!;
+    return (await x as number);
+    return <string> await x;
+}
+async function allowedInsideTryCatch() {
+    try {
+        return await x;
+    } catch (e) {
+        // handle error
+        return x; // not allowed in catch when there is no finally
+    }
+}
+async function allowedInsideTryFinally() {
+    try {
+        return await x;
+    } finally {
+        // do cleanup, close connection for example
+        return x; // not allowed in finally at all
+    }
+}
+async function allowedInsideCatchFinally() {
+    try {
+        return await x;
+    } catch (e) {
+        return await x;
+    } finally {
+        return x;
+    }
+}
+async function nestedTryStatements() {
+    try {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return await foobar;
+        }
+    } catch(e) {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return await foobar;
+        }
+    } finally {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return x;
+        }
+    }
+}
+
+async function handleParens() {
+    return (x);
+}
+async function handleBinaryExpression() {
+    return await foo || x;
+    return await foo && x;
+    return await foo, x;
+    return await foo + await bar;
+}
+async function handleConditionalExpression() {
+    return await foo ? bar : baz;
+}
+
+throw await x;
+await x;
+return x;
+

test/rules/no-return-await/test.ts.lint

@@ -0,0 +1,94 @@
+async function foo() {
+    return await x;
+           ~~~~~ [0]
+}
+(async () => await x);
+             ~~~~~ [0]
+
+async function allowTypeAssertion() {
+    return (await x)!;
+    return (await x as number);
+    return <string> await x;
+}
+async function allowedInsideTryCatch() {
+    try {
+        return await x;
+    } catch (e) {
+        // handle error
+        return await x; // not allowed in catch when there is no finally
+               ~~~~~ [0]
+    }
+}
+async function allowedInsideTryFinally() {
+    try {
+        return await x;
+    } finally {
+        // do cleanup, close connection for example
+        return await x; // not allowed in finally at all
+               ~~~~~ [0]
+    }
+}
+async function allowedInsideCatchFinally() {
+    try {
+        return await x;
+    } catch (e) {
+        return await x;
+    } finally {
+        return await x;
+               ~~~~~ [0]
+    }
+}
+async function nestedTryStatements() {
+    try {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return await foobar;
+        }
+    } catch(e) {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return await foobar;
+        }
+    } finally {
+        try {
+            return await x;
+        } catch (e) {
+            return await x;
+        } finally {
+            return await x;
+                   ~~~~~ [0]
+        }
+    }
+}
+
+async function handleParens() {
+    return (await x);
+            ~~~~~ [0]
+}
+async function handleBinaryExpression() {
+    return await foo || await x;
+                        ~~~~~ [0]
+    return await foo && await x;
+                        ~~~~~ [0]
+    return await foo, await x;
+                      ~~~~~ [0]
+    return await foo + await bar;
+}
+async function handleConditionalExpression() {
+    return await foo ? await bar : await baz;
+                       ~~~~~ [0]
+                                   ~~~~~ [0]
+}
+
+throw await x;
+await x;
+return await x;
+       ~~~~~ [0]
+
+[0]: Unnecessary 'await'.

test/rules/no-return-await/tslint.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "no-return-await": true
+  }
+}