Use singular array element variable name in autofix for no-for-loop…

… rule (#745)

package.json

@@ -40,6 +40,7 @@
 		"eslint-utils": "^2.0.0",
 		"import-modules": "^2.0.0",
 		"lodash": "^4.17.15",
+		"pluralize": "^8.0.0",
 		"read-pkg-up": "^7.0.1",
 		"regexp-tree": "^0.1.21",
 		"reserved-words": "^0.1.2",

rules/no-for-loop.js

@@ -3,6 +3,7 @@ const getDocumentationUrl = require('./utils/get-documentation-url');
 const isLiteralValue = require('./utils/is-literal-value');
 const {flatten} = require('lodash');
 const avoidCapture = require('./utils/avoid-capture');
+const {singular} = require('pluralize');
 
 const defaultElementName = 'element';
 const isLiteralZero = node => isLiteralValue(node, 0);
@@ -267,6 +268,13 @@ const getChildScopesRecursive = scope => [
 	...flatten(scope.childScopes.map(scope => getChildScopesRecursive(scope)))
 ];
 
+const getSingularName = originalName => {
+	const singularName = singular(originalName);
+	if (singularName !== originalName) {
+		return singularName;
+	}
+};
+
 const create = context => {
 	const sourceCode = context.getSourceCode();
 	const {scopeManager} = sourceCode;
@@ -342,7 +350,7 @@ const create = context => {
 
 					const index = indexIdentifierName;
 					const element = elementIdentifierName ||
-						avoidCapture(defaultElementName, getChildScopesRecursive(bodyScope), context.parserOptions.ecmaVersion);
+						avoidCapture(getSingularName(arrayIdentifierName) || defaultElementName, getChildScopesRecursive(bodyScope), context.parserOptions.ecmaVersion);
 					const array = arrayIdentifierName;
 
 					let declarationElement = element;

test/no-for-loop.js

@@ -649,6 +649,57 @@ ruleTester.run('no-for-loop', rule, {
 				console.log(element);
 				console.log(element_);
 			}
+		`),
+
+		// Singularization:
+		...[
+			['plugin', 'plugins'], // Simple
+			['person', 'people'], // Irregular
+			['girlsAndBoy', 'girlsAndBoys'], // Multiple plurals
+			['largeCity', 'largeCities'], // CamelCase
+			['LARGE_CITY', 'LARGE_CITIES'], // Caps, snake_case
+			['element', 'news'], // No singular version, ends in s
+			['element', 'list'] // No singular version
+		].map(([elementName, arrayName]) =>
+			testCase(
+				`for(const i = 0; i < ${arrayName}.length; i++) {console.log(${arrayName}[i])}`,
+				`for(const ${elementName} of ${arrayName}) {console.log(${elementName})}`
+			)
+		),
+
+		// Singularization (avoid using reserved JavaScript keywords):
+		testCase(outdent`
+			for (let i = 0; i < cases.length; i++) {
+				console.log(cases[i]);
+			}
+		`, outdent`
+			for (const case_ of cases) {
+				console.log(case_);
+			}
+		`),
+		// Singularization (avoid variable name collision):
+		testCase(outdent`
+			for (let i = 0; i < cities.length; i++) {
+				console.log(cities[i]);
+				const city = foo();
+				console.log(city);
+			}
+		`, outdent`
+			for (const city_ of cities) {
+				console.log(city_);
+				const city = foo();
+				console.log(city);
+			}
+		`),
+		// Singularization (uses i):
+		testCase(outdent`
+			for (let i = 0; i < cities.length; i++) {
+				console.log(i, cities[i]);
+			}
+		`, outdent`
+			for (const [i, city] of cities.entries()) {
+				console.log(i, city);
+			}
 		`)
 	]
 });