eslint: fix no-return-assign (wikimedia-gadgets#2103)

* eslint: fix no-return-assign

manual fix

* use sum++ instead of ternary, easier to read

* use .filter() instead of .reduce()

* fix linter error

* add test

.eslintrc.json

@@ -54,7 +54,6 @@
 		"no-jquery/no-sizzle": "warn",
 		"no-loop-func": "warn",
 		"no-new": "warn",
-		"no-return-assign": "warn",
 		"no-script-url": "warn",
 		"no-unused-expressions": "warn",
 		"no-use-before-define": "warn",

modules/twinkletag.js

@@ -2009,7 +2009,7 @@ Twinkle.tag.callbacks = {
  * @return {true|undefined}
  */
 Twinkle.tag.checkIncompatible = function(incompatibleTags, tagsToCheck, extraMessage = null) {
-	const count = incompatibleTags.reduce((sum, tag) => sum += tagsToCheck.indexOf(tag) !== -1, 0);
+	const count = incompatibleTags.filter((tag) => tagsToCheck.includes(tag)).length;
 	if (count > 1) {
 		const incompatibleTagsString = '{{' + incompatibleTags.join('}}, {{') + '}}';
 		let message = 'Please select only one of: ' + incompatibleTagsString + '.';

tests/twinkletag.test.js

@@ -1,6 +1,13 @@
 describe('modules/twinkletag', () => {
 	describe('checkIncompatible', () => {
-		test('no conflicts', () => {
+		test('no conflicts, 0 tags to check', () => {
+			const incompatibleTags = ['Bad GIF', 'Bad JPEG', 'Bad SVG', 'Bad format'];
+			const tagsToCheck = [];
+			const expected = undefined;
+			expect(Twinkle.tag.checkIncompatible(incompatibleTags, tagsToCheck)).toBe(expected);
+		});
+
+		test('no conflicts, 1 tag to check', () => {
 			const incompatibleTags = ['Bad GIF', 'Bad JPEG', 'Bad SVG', 'Bad format'];
 			const tagsToCheck = ['Better source requested'];
 			const expected = undefined;