feat: use singular array element variable name in autofix for `no-for…

…-loop` rule

If the loop is iterating an array with a plural name (such as `plugins`), the generated element variable in the autofix could use the singular version of the array name (`plugin`), instead of just the generic name `element`.

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

@@ -2,6 +2,8 @@
 const getDocumentationUrl = require('./utils/get-documentation-url');
 const isLiteralValue = require('./utils/is-literal-value');
 const {flatten} = require('lodash');
+const {singular} = require('pluralize');
+const reservedWords = require('reserved-words');
 
 const defaultElementName = 'element';
 const isLiteralZero = node => isLiteralValue(node, 0);
@@ -261,6 +263,24 @@ const getReferencesInChildScopes = (scope, name) => {
 	];
 };
 
+const getSingularName = originalName => {
+	const singularName = singular(originalName);
+	if (
+		!singularName ||
+		singularName === originalName
+	) {
+		// Bail if we can't produce a singular name that differs from the original name.
+		return;
+	}
+
+	if (reservedWords.check(singularName, '6', true)){ // 6 is the latest version understood by `reservedWords`.
+		// Add suffix to avoid using a reserved JavaScript keyword.
+		return `${singularName}_`;
+	}
+
+	return singularName;
+};
+
 const create = context => {
 	const sourceCode = context.getSourceCode();
 	const {scopeManager} = sourceCode;
@@ -335,7 +355,7 @@ const create = context => {
 					const shouldGenerateIndex = isIndexVariableUsedElsewhereInTheLoopBody(indexVariable, bodyScope, arrayIdentifierName);
 
 					const index = indexIdentifierName;
-					const element = elementIdentifierName || defaultElementName;
+					const element = elementIdentifierName || getSingularName(arrayIdentifierName) || defaultElementName;
 					const array = arrayIdentifierName;
 
 					let declarationElement = element;

test/no-for-loop.js

@@ -482,6 +482,46 @@ ruleTester.run('no-for-loop', rule, {
 				const [ a, b ] = element;
 				console.log(a, b, element);
 			}
+		`),
+		// Singularization (simple case):
+		testCase(outdent`
+			for (let i = 0; i < plugins.length; i++) {
+				console.log(plugins[i]);
+			}
+		`, outdent`
+			for (const plugin of plugins) {
+				console.log(plugin);
+			}
+		`),
+		// Singularization (irregular case):
+		testCase(outdent`
+			for (let i = 0; i < people.length; i++) {
+				console.log(people[i]);
+			}
+		`, outdent`
+			for (const person of people) {
+				console.log(person);
+			}
+		`),
+		// 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 (capital letters, multiple words):
+		testCase(outdent`
+			for (let i = 0; i < LARGE_CITIES.length; i++) {
+				console.log(LARGE_CITIES[i]);
+			}
+		`, outdent`
+			for (const LARGE_CITY of LARGE_CITIES) {
+				console.log(LARGE_CITY);
+			}
 		`)
 	]
 });