✨ add node/no-exports-assign rule (fixes #175)

Author: mysticatea

README.md

@@ -64,6 +64,8 @@ $ npm install --save-dev eslint eslint-plugin-node
 
 | Rule ID | Description |    |
 |:--------|:------------|:--:|
+| [node/no-callback-literal](./docs/rules/no-callback-literal.md) | ensure Node.js-style error-first callback pattern is followed |  |
+| [node/no-exports-assign](./docs/rules/no-exports-assign.md) | disallow the assignment to `exports` |  |
 | [node/no-extraneous-import](./docs/rules/no-extraneous-import.md) | disallow `import` declarations which import extraneous modules | ⭐️ |
 | [node/no-extraneous-require](./docs/rules/no-extraneous-require.md) | disallow `require()` expressions which import extraneous modules | ⭐️ |
 | [node/no-missing-import](./docs/rules/no-missing-import.md) | disallow `import` declarations which import non-existence modules | ⭐️ |

docs/rules/no-callback-literal.md

@@ -1,6 +1,5 @@
 # node/no-callback-literal
-
-> Ensures the Node.js error-first callback pattern is followed
+> ensure Node.js-style error-first callback pattern is followed
 
 When invoking a callback function which uses the Node.js error-first callback pattern, all of your errors should either use the `Error` class or a subclass of it. It is also acceptable to use `undefined` or `null` if there is no error.
 

docs/rules/no-exports-assign.md

@@ -0,0 +1,45 @@
+# node/no-exports-assign
+> disallow the assignment to `exports`
+
+To assign to `exports` variable would not work as expected.
+
+```js
+// This assigned object is not exported.
+// You need to use `module.exports = { ... }`.
+exports = {
+    foo: 1
+}
+```
+
+## 📖 Rule Details
+
+This rule is aimed at disallowing `exports = {}`, but allows `module.exports = exports = {}` to avoid conflict with [node/exports-style](./exports-style.md) rule's `allowBatchAssign` option.
+
+👍 Examples of **correct** code for this rule:
+
+```js
+/*eslint node/no-exports-assign: error */
+
+module.exports.foo = 1
+exports.bar = 2
+
+module.exports = {}
+
+// allows `exports = {}` if along with `module.exports =`
+module.exports = exports = {}
+exports = module.exports = {}
+```
+
+👎 Examples of **incorrect** code for this rule:
+
+```js
+/*eslint node/no-exports-assign: error */
+
+exports = {}
+```
+
+
+## 🔎 Implementation
+
+- [Rule source](../../lib/rules/no-exports-assign.js)
+- [Test source](../../tests/lib/rules/no-exports-assign.js)

lib/index.js

@@ -12,7 +12,9 @@ module.exports = {
     rules: {
         "exports-style": require("./rules/exports-style"),
         "file-extension-in-import": require("./rules/file-extension-in-import"),
+        "no-callback-literal": require("./rules/no-callback-literal"),
         "no-deprecated-api": require("./rules/no-deprecated-api"),
+        "no-exports-assign": require("./rules/no-exports-assign"),
         "no-extraneous-import": require("./rules/no-extraneous-import"),
         "no-extraneous-require": require("./rules/no-extraneous-require"),
         "no-missing-import": require("./rules/no-missing-import"),

lib/rules/no-exports-assign.js

@@ -0,0 +1,75 @@
+/**
+ * @author Toru Nagashima <https://github.com/mysticatea>
+ * See LICENSE file in root directory for full license.
+ */
+"use strict"
+
+const { findVariable } = require("eslint-utils")
+
+function isExports(node, scope) {
+    let variable = null
+
+    return (
+        node != null &&
+        node.type === "Identifier" &&
+        node.name === "exports" &&
+        (variable = findVariable(scope, node)) != null &&
+        variable.scope.type === "global"
+    )
+}
+
+function isModuleExports(node, scope) {
+    let variable = null
+
+    return (
+        node != null &&
+        node.type === "MemberExpression" &&
+        !node.computed &&
+        node.object.type === "Identifier" &&
+        node.object.name === "module" &&
+        node.property.type === "Identifier" &&
+        node.property.name === "exports" &&
+        (variable = findVariable(scope, node.object)) != null &&
+        variable.scope.type === "global"
+    )
+}
+
+module.exports = {
+    meta: {
+        docs: {
+            description: "disallow the assignment to `exports`",
+            category: "Possible Errors",
+            recommended: false,
+            url:
+                "https://github.com/mysticatea/eslint-plugin-node/blob/v9.2.0/docs/rules/no-exports-assign.md",
+        },
+        fixable: null,
+        messages: {
+            forbidden:
+                "Unexpected assignment to 'exports' variable. Use 'module.exports' instead.",
+        },
+        schema: [],
+        type: "problem",
+    },
+    create(context) {
+        return {
+            AssignmentExpression(node) {
+                const scope = context.getScope()
+                if (
+                    !isExports(node.left, scope) ||
+                    // module.exports = exports = {}
+                    (node.parent.type === "AssignmentExpression" &&
+                        node.parent.right === node &&
+                        isModuleExports(node.parent.left, scope)) ||
+                    // exports = module.exports = {}
+                    (node.right.type === "AssignmentExpression" &&
+                        isModuleExports(node.right.left, scope))
+                ) {
+                    return
+                }
+
+                context.report({ node, messageId: "forbidden" })
+            },
+        }
+    },
+}

tests/lib/rules/no-exports-assign.js

@@ -0,0 +1,31 @@
+/**
+ * @author Toru Nagashima <https://github.com/mysticatea>
+ * See LICENSE file in root directory for full license.
+ */
+"use strict"
+
+const { RuleTester } = require("eslint")
+const rule = require("../../../lib/rules/no-exports-assign.js")
+
+new RuleTester({
+    globals: {
+        exports: "writable",
+        module: "readonly",
+    },
+}).run("no-exports-assign", rule, {
+    valid: [
+        "module.exports.foo = 1",
+        "exports.bar = 1",
+        "module.exports = exports = {}",
+        "exports = module.exports = {}",
+        "function f(exports) { exports = {} }",
+    ],
+    invalid: [
+        {
+            code: "exports = {}",
+            errors: [
+                "Unexpected assignment to 'exports' variable. Use 'module.exports' instead.",
+            ],
+        },
+    ],
+})