New: no-unused-labels rule (fixes #5052)

conf/eslint.json

@@ -107,6 +107,7 @@
         "no-unneeded-ternary": 0,
         "no-unreachable": 2,
         "no-unused-expressions": 0,
+        "no-unused-labels": 0,
         "no-unused-vars": 2,
         "no-use-before-define": 0,
         "no-useless-call": 0,

docs/rules/README.md

@@ -93,6 +93,7 @@ These are rules designed to prevent you from making mistakes. They either prescr
 * [no-throw-literal](no-throw-literal.md) - restrict what can be thrown as an exception
 * [no-unmodified-loop-condition](no-unmodified-loop-condition.md) - disallow unmodified conditions of loops
 * [no-unused-expressions](no-unused-expressions.md) - disallow usage of expressions in statement position
+* [no-unused-labels](no-unused-labels.md) - disallow unused labels
 * [no-useless-call](no-useless-call.md) - disallow unnecessary `.call()` and `.apply()`
 * [no-useless-concat](no-useless-concat.md) - disallow unnecessary concatenation of literals or template literals
 * [no-void](no-void.md) - disallow use of the `void` operator

docs/rules/no-empty-label.md

@@ -26,3 +26,13 @@ for (var i=10; i; i--) {
     // ...
 }
 ```
+
+## When Not To Use It
+
+If you don't want to be notified about usage of labels, then it's safe to disable this rule.
+
+## Related Rules
+
+* [no-labels](./no-labels.md)
+* [no-label-var](./no-label-var.md)
+* [no-unused-labels](./no-unused-labels.md)

docs/rules/no-label-var.md

@@ -37,6 +37,16 @@ q:
 }
 ```
 
+## When Not To Use It
+
+If you don't want to be notified about usage of labels, then it's safe to disable this rule.
+
+## Related Rules
+
+* [no-empty-label](./no-empty-label.md)
+* [no-labels](./no-labels.md)
+* [no-unused-labels](./no-unused-labels.md)
+
 ## Further Reading
 
 * ['{a}' is a statement label](http://jslinterrors.com/a-is-a-statement-label/)

docs/rules/no-labels.md

@@ -62,3 +62,9 @@ while (true) {
 ## When Not To Use It
 
 If you need to use labeled statements, then you can safely disable this rule.
+
+## Related Rules
+
+* [no-empty-label](./no-empty-label.md)
+* [no-label-var](./no-label-var.md)
+* [no-unused-labels](./no-unused-labels.md)

docs/rules/no-unused-labels.md

@@ -0,0 +1,68 @@
+# Disallow Unused Labels (no-unused-labels)
+
+Labels that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring.
+
+```js
+OUTER_LOOP:
+for (const student of students) {
+    if (checkScores(student.scores)) {
+        continue;
+    }
+    doSomething(student);
+}
+```
+
+In this case, probably removing `OUTER_LOOP:` had been forgotten.
+Such labels take up space in the code and can lead to confusion by readers.
+
+## Rule Details
+
+This rule is aimed at eliminating unused labels.
+
+The following patterns are considered problems:
+
+```js
+/*eslint no-unused-labels: 2*/
+
+A: var foo = 0;  /*error 'A:' is defined but never used.*/
+
+B: {             /*error 'B:' is defined but never used.*/
+    foo();
+}
+
+C:               /*error 'C:' is defined but never used.*/
+for (let i = 0; i < 10; ++i) {
+    foo();
+}
+```
+
+The following patterns are considered not problems:
+
+```js
+/*eslint no-unused-labels: 2*/
+
+A: {
+    if (foo()) {
+        break A;
+    }
+    bar();
+}
+
+B:
+for (let i = 0; i < 10; ++i) {
+    if (foo()) {
+        break B;
+    }
+    bar();
+}
+```
+
+## When Not To Use It
+
+If you don't want to be notified about unused labels, then it's safe to disable this rule.
+
+## Related Rules
+
+* [no-empty-label](./no-empty-label.md)
+* [no-labels](./no-labels.md)
+* [no-label-var](./no-label-var.md)

lib/rules/no-unused-labels.js

@@ -0,0 +1,81 @@
+/**
+ * @fileoverview Rule to disallow unused labels.
+ * @author Toru Nagashima
+ * @copyright 2016 Toru Nagashima. All rights reserved.
+ * See LICENSE file in root directory for full license.
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function(context) {
+    var scopeInfo = null;
+
+    /**
+     * Adds a scope info to the stack.
+     *
+     * @param {ASTNode} node - A node to add. This is a LabeledStatement.
+     * @returns {void}
+     */
+    function enterLabeledScope(node) {
+        scopeInfo = {
+            label: node.label.name,
+            used: false,
+            upper: scopeInfo
+        };
+    }
+
+    /**
+     * Removes the top of the stack.
+     * At the same time, this reports the label if it's never used.
+     *
+     * @param {ASTNode} node - A node to report. This is a LabeledStatement.
+     * @returns {void}
+     */
+    function exitLabeledScope(node) {
+        if (!scopeInfo.used) {
+            context.report({
+                node: node.label,
+                message: "'{{name}}:' is defined but never used.",
+                data: node.label
+            });
+        }
+
+        scopeInfo = scopeInfo.upper;
+    }
+
+    /**
+     * Marks the label of a given node as used.
+     *
+     * @param {ASTNode} node - A node to mark. This is a BreakStatement or
+     *      ContinueStatement.
+     * @returns {void}
+     */
+    function markAsUsed(node) {
+        if (!node.label) {
+            return;
+        }
+
+        var label = node.label.name;
+        var info = scopeInfo;
+        while (info) {
+            if (info.label === label) {
+                info.used = true;
+                break;
+            }
+            info = info.upper;
+        }
+    }
+
+    return {
+        "LabeledStatement": enterLabeledScope,
+        "LabeledStatement:exit": exitLabeledScope,
+        "BreakStatement": markAsUsed,
+        "ContinueStatement": markAsUsed
+    };
+};
+
+module.exports.schema = [];

tests/lib/rules/no-unused-labels.js

@@ -0,0 +1,47 @@
+/**
+ * @fileoverview Tests for no-unused-labels rule.
+ * @author Toru Nagashima
+ * @copyright 2016 Toru Nagashima. All rights reserved.
+ * See LICENSE file in root directory for full license.
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var rule = require("../../../lib/rules/no-unused-labels"),
+    RuleTester = require("../../../lib/testers/rule-tester");
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+var ruleTester = new RuleTester();
+ruleTester.run("no-unused-labels", rule, {
+    valid: [
+        "A: break A;",
+        "A: { foo(); break A; bar(); }",
+        "A: if (a) { foo(); if (b) break A; bar(); }",
+        "A: for (var i = 0; i < 10; ++i) { foo(); if (a) break A; bar(); }",
+        "A: for (var i = 0; i < 10; ++i) { foo(); if (a) continue A; bar(); }",
+        "A: { B: break B; C: for (var i = 0; i < 10; ++i) { foo(); if (a) break A; if (c) continue C; bar(); } }",
+        "A: { var A = 0; console.log(A); break A; console.log(A); }"
+    ],
+    invalid: [
+        {code: "A: var foo = 0;", errors: ["'A:' is defined but never used."]},
+        {code: "A: { foo(); bar(); }", errors: ["'A:' is defined but never used."]},
+        {code: "A: if (a) { foo(); bar(); }", errors: ["'A:' is defined but never used."]},
+        {code: "A: for (var i = 0; i < 10; ++i) { foo(); if (a) break; bar(); }", errors: ["'A:' is defined but never used."]},
+        {code: "A: for (var i = 0; i < 10; ++i) { foo(); if (a) continue; bar(); }", errors: ["'A:' is defined but never used."]},
+        {code: "A: for (var i = 0; i < 10; ++i) { B: break A; }", errors: ["'B:' is defined but never used."]},
+        {code: "A: { var A = 0; console.log(A); }", errors: ["'A:' is defined but never used."]}
+
+        // Below is fatal errors.
+        // "A: break B",
+        // "A: function foo() { break A; }",
+        // "A: class Foo { foo() { break A; } }",
+        // "A: { A: { break A; } }"
+    ]
+});