New rule: disallowUnusedVariables

Fixes #2076
Closes gh-2199

lib/rules/disallow-unused-variables.js

@@ -0,0 +1,140 @@
+/**
+ * Disallows unused variables defined with var, let or const.
+ *
+ * Types: `Boolean`
+ *
+ * Values: `true`
+ *
+ * #### Example
+ *
+ * ```js
+ * "disallowUnusedVariables": true
+ * ```
+ *
+ * ##### Valid
+ *
+ * ```js
+ * var x=1;
+ *
+ * function getX() {
+ *     return x;
+ * }
+ *
+ * ```
+ *
+ * ##### Invalid
+ *
+ * ```js
+ * var x=1;
+ *
+ * function getX() {
+ *     return true;
+ * }
+ *
+ * ```
+ */
+
+var assert = require('assert');
+
+module.exports = function() {};
+
+module.exports.prototype = {
+    configure: function(options) {
+        assert(
+            options === true,
+            this.getOptionName() + ' option requires a true value or should be removed'
+        );
+    },
+
+    getOptionName: function() {
+        return 'disallowUnusedVariables';
+    },
+
+    check: function(file, errors) {
+        var program = file.getProgram();
+        var variableList = [];
+        var nodesToCheck = [];
+        var unusedNodes = [];
+
+        function reportError(node) {
+            errors.add('Variable `' + node.name + '` is not used', node);
+        }
+
+        function isVariableGood(variable) {
+            var parentCheck = function(node) {
+                if (node.parentElement) {
+                    if (node.parentElement.type === 'VariableDeclaration') {
+                        var grandparentElement = node.parentElement.parentElement;
+
+                        return grandparentElement.type !== 'ExportNamedDeclaration';
+                    } else {
+                        return parentCheck(node.parentElement);
+                    }
+                } else {
+                    return false;
+                }
+            }
+
+            var useVariable = variable.definitions.some(function checkVariableDefinition(definition) {
+                return parentCheck(definition.node);
+            });
+
+
+            return useVariable;
+        }
+
+        function getVariablesInAllScopes(scope) {
+            var variableList = [];
+
+            var iterateChildScopes = function(scope) {
+                scope.variables.forEach(function(variable) {
+                    variableList.push(variable);
+                });
+
+                scope.childScopes.forEach(function(childScope){
+                    return iterateChildScopes(childScope);
+                });
+            };
+
+            iterateChildScopes(scope);
+
+            return variableList;
+        }
+
+        // Get all variables in all scopes.
+        variableList = getVariablesInAllScopes(file.getScopes().acquire(program));
+
+        // Check if variables are what we want to check..
+        variableList.reduce(function(acc, variable) {
+            if (isVariableGood(variable)) {
+                acc.push(variable);
+            }
+
+            return acc;
+        }, nodesToCheck);
+
+        // Check if variables are used.
+        nodesToCheck.reduce(function checkVariableReferences(acc, variable) {
+            if (variable.references.length === 1) {
+                variable.definitions.forEach(function addUnusedVariable(definition) {
+                    acc.push(definition.node);
+                });
+            }
+
+            return acc;
+        }, unusedNodes);
+
+        unusedNodes.forEach(reportError);
+    },
+
+    _fix: function(file, error) {
+        var node = error.element;
+
+        while(node.type !== 'VariableDeclaration') {
+            node = node.parentElement;
+        }
+
+        node.parentElement.removeChild(node);
+        error.fixed = true;
+    }
+};