New: new rule no-constructor-return (fixes #12481)

docs/rules/no-constructor-return.md

@@ -0,0 +1,52 @@
+# Disallow returning value in constructor (no-constructor-return)
+
+In JavaScript returning value in constructor is allowed, however this may be a mistake. Forbidding this pattern prevents mistake resulting from unfamiliarity with the language or copy-paste error.
+
+Note that returning nothing with flow control is allowed.
+
+## Rule Details
+
+This rule disallows return statement in constructor of a class.
+
+Examples of **incorrect** code for this rule:
+
+```js
+/*eslint no-constructor-return: "error"*/
+
+class A {
+    constructor(a) {
+        this.a = a;
+        return a;
+    }
+}
+
+class B {
+    constructor(f) {
+        if (!f) {
+            return 'falsy';
+        }
+    }
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/*eslint no-constructor-return: "error"*/
+
+class C {
+    constructor(c) {
+        this.c = c;
+    }
+}
+
+class D {
+    constructor(f) {
+        if (!f) {
+            return;  // Flow control.
+        }
+
+        f();
+    }
+}
+```

lib/rules/index.js

@@ -101,6 +101,7 @@ module.exports = new LazyLoadingRuleMap(Object.entries({
     "no-console": () => require("./no-console"),
     "no-const-assign": () => require("./no-const-assign"),
     "no-constant-condition": () => require("./no-constant-condition"),
+    "no-constructor-return": () => require("./no-constructor-return"),
     "no-continue": () => require("./no-continue"),
     "no-control-regex": () => require("./no-control-regex"),
     "no-debugger": () => require("./no-debugger"),

lib/rules/no-constructor-return.js

@@ -0,0 +1,61 @@
+/**
+ * @fileoverview Rule to disallow returning value from constructor.
+ * @author Pig Fang <https://github.com/g-plane>
+ */
+
+"use strict";
+
+const astUtils = require("./utils/ast-utils");
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        type: "suggestion",
+
+        docs: {
+            description: "disallow returning value from constructor",
+            category: "Best Practices",
+            recommended: false,
+            url: "https://eslint.org/docs/rules/no-constructor-return"
+        },
+
+        schema: {},
+
+        fixable: null,
+
+        messages: {
+            unexpected: "Unexpected return statement in constructor."
+        }
+    },
+
+    create(context) {
+        return {
+            "MethodDefinition[kind='constructor'] ReturnStatement"(node) {
+                const ancestors = context.getAncestors().reverse();
+                const ctorIndex = ancestors.findIndex(
+                    ancestor => ancestor.type === "MethodDefinition" && ancestor.kind === "constructor"
+                );
+                const upperFnIndex = ancestors.findIndex(astUtils.isFunction);
+
+                if (~upperFnIndex && upperFnIndex < ctorIndex - 1) {
+                    return;
+                }
+
+                if (
+                    ancestors[2] &&
+                    ancestors[2].type === "MethodDefinition" &&
+                    ancestors[2].kind === "constructor" ||
+                    node.argument
+                ) {
+                    context.report({
+                        node,
+                        messageId: "unexpected"
+                    });
+                }
+            }
+        };
+    }
+};

tests/lib/rules/no-constructor-return.js

@@ -0,0 +1,49 @@
+/**
+ * @fileoverview Tests for no-constructor-return rule.
+ * @author Pig Fang <https://github.com/g-plane>
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const rule = require("../../../lib/rules/no-constructor-return"),
+    { RuleTester } = require("../../../lib/rule-tester");
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 2015 } });
+
+const errors = [{ type: "ReturnStatement", messageId: "unexpected" }];
+
+ruleTester.run("no-constructor-return", rule, {
+    valid: [
+        "class C {  }",
+        "class C { constructor() {} }",
+        "class C { constructor() { let v } }",
+        "class C { method() { return '' } }",
+        "class C { get value() { return '' } }",
+        "class C { constructor(a) { if (!a) { return } else { a() } } }",
+        "class C { constructor() { function fn() { return true } } }",
+        "class C { constructor() { this.fn = function () { return true } } }",
+        "class C { constructor() { this.fn = () => { return true } } }"
+    ],
+    invalid: [
+        {
+            code: "class C { constructor() { return } }",
+            errors
+        },
+        {
+            code: "class C { constructor() { return '' } }",
+            errors
+        },
+        {
+            code: "class C { constructor(a) { if (!a) { return '' } else { a() } } }",
+            errors
+        }
+    ]
+});

tools/rule-types.json

@@ -88,6 +88,7 @@
     "no-console": "suggestion",
     "no-const-assign": "problem",
     "no-constant-condition": "problem",
+    "no-constructor-return": "problem",
     "no-continue": "suggestion",
     "no-control-regex": "problem",
     "no-debugger": "problem",