Add consistent-empty-array-spread rule (#2349)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>
sindresorhus · May 10, 2024 · 8d7954c · 8d7954c
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diff --git a/docs/rules/consistent-empty-array-spread.md b/docs/rules/consistent-empty-array-spread.md
@@ -0,0 +1,42 @@
+# Prefer consistent types when spreading a ternary in an array literal
+
+💼 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs-eslintconfigjs).
+
+🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
+
+<!-- end auto-generated rule header -->
+<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
+
+When spreading a ternary in an array, we can use both `[]` and `''` as fallbacks, but it's better to have consistent types in both branches.
+
+## Fail
+
+```js
+const array = [
+	a,
+	...(foo ? [b, c] : ''),
+];
+```
+
+```js
+const array = [
+	a,
+	...(foo ? 'bc' : []),
+];
+```
+
+## Pass
+
+```js
+const array = [
+	a,
+	...(foo ? [b, c] : []),
+];
+```
+
+```js
+const array = [
+	a,
+	...(foo ? 'bc' : ''),
+];
+```
diff --git a/readme.md b/readme.md
@@ -113,6 +113,7 @@ If you don't use the preset, ensure you use the same `env` and `parserOptions` c
 | [better-regex](docs/rules/better-regex.md)                                                       | Improve regexes by making them shorter, consistent, and safer.                                                                                                                                                    | ✅  | 🔧 |    |
 | [catch-error-name](docs/rules/catch-error-name.md)                                               | Enforce a specific parameter name in catch clauses.                                                                                                                                                               | ✅  | 🔧 |    |
 | [consistent-destructuring](docs/rules/consistent-destructuring.md)                               | Use destructured variables over properties.                                                                                                                                                                       |    | 🔧 | 💡 |
+| [consistent-empty-array-spread](docs/rules/consistent-empty-array-spread.md)                     | Prefer consistent types when spreading a ternary in an array literal.                                                                                                                                             | ✅  | 🔧 |    |
 | [consistent-function-scoping](docs/rules/consistent-function-scoping.md)                         | Move function definitions to the highest possible scope.                                                                                                                                                          | ✅  |    |    |
 | [custom-error-definition](docs/rules/custom-error-definition.md)                                 | Enforce correct `Error` subclassing.                                                                                                                                                                              |    | 🔧 |    |
 | [empty-brace-spaces](docs/rules/empty-brace-spaces.md)                                           | Enforce no spaces between braces.                                                                                                                                                                                 | ✅  | 🔧 |    |

diff --git a/rules/consistent-empty-array-spread.js b/rules/consistent-empty-array-spread.js
@@ -0,0 +1,126 @@
+'use strict';
+const {getStaticValue} = require('@eslint-community/eslint-utils');
+
+const MESSAGE_ID = 'consistent-empty-array-spread';
+const messages = {
+	[MESSAGE_ID]: 'Prefer using empty {{replacementDescription}} since the {{anotherNodePosition}} is {{anotherNodeDescription}}.',
+};
+
+const isEmptyArrayExpression = node =>
+	node.type === 'ArrayExpression'
+	&& node.elements.length === 0;
+
+const isEmptyStringLiteral = node =>
+	node.type === 'Literal'
+	&& node.value === '';
+
+const isString = (node, context) => {
+	const staticValueResult = getStaticValue(node, context.sourceCode.getScope(node));
+	return typeof staticValueResult?.value === 'string';
+};
+
+const isArray = (node, context) => {
+	if (node.type === 'ArrayExpression') {
+		return true;
+	}
+
+	const staticValueResult = getStaticValue(node, context.sourceCode.getScope(node));
+	return Array.isArray(staticValueResult?.value);
+};
+
+const cases = [
+	{
+		oneSidePredicate: isEmptyStringLiteral,
+		anotherSidePredicate: isArray,
+		anotherNodeDescription: 'an array',
+		replacementDescription: 'array',
+		replacementCode: '[]',
+	},
+	{
+		oneSidePredicate: isEmptyArrayExpression,
+		anotherSidePredicate: isString,
+		anotherNodeDescription: 'a string',
+		replacementDescription: 'string',
+		replacementCode: '\'\'',
+	},
+];
+
+function createProblem({
+	problemNode,
+	anotherNodePosition,
+	anotherNodeDescription,
+	replacementDescription,
+	replacementCode,
+}) {
+	return {
+		node: problemNode,
+		messageId: MESSAGE_ID,
+		data: {
+			replacementDescription,
+			anotherNodePosition,
+			anotherNodeDescription,
+		},
+		fix: fixer => fixer.replaceText(problemNode, replacementCode),
+	};
+}
+
+function getProblem(conditionalExpression, context) {
+	const {
+		consequent,
+		alternate,
+	} = conditionalExpression;
+
+	for (const problemCase of cases) {
+		const {
+			oneSidePredicate,
+			anotherSidePredicate,
+		} = problemCase;
+
+		if (oneSidePredicate(consequent, context) && anotherSidePredicate(alternate, context)) {
+			return createProblem({
+				...problemCase,
+				problemNode: consequent,
+				anotherNodePosition: 'alternate',
+			});
+		}
+
+		if (oneSidePredicate(alternate, context) && anotherSidePredicate(consequent, context)) {
+			return createProblem({
+				...problemCase,
+				problemNode: alternate,
+				anotherNodePosition: 'consequent',
+			});
+		}
+	}
+}
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => ({
+	* ArrayExpression(arrayExpression) {
+		for (const element of arrayExpression.elements) {
+			if (
+				element?.type !== 'SpreadElement'
+				|| element.argument.type !== 'ConditionalExpression'
+			) {
+				continue;
+			}
+
+			yield getProblem(element.argument, context);
+		}
+	},
+});
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Prefer consistent types when spreading a ternary in an array literal.',
+			recommended: true,
+		},
+		fixable: 'code',
+
+		messages,
+	},
+};
diff --git a/scripts/template/rule.js.jst b/scripts/template/rule.js.jst
@@ -17,15 +17,14 @@ const messages = {
 };
 <% } %>
 
-const selector = [
-	'Literal',
-	'[value="unicorn"]',
-].join('');
-
 /** @param {import('eslint').Rule.RuleContext} context */
 const create = context => {
 	return {
-		[selector](node) {
+		Literal(node) {
+			if (node.value !== 'unicorn') {
+				return;
+			}
+
 			return {
 				node,
 				messageId: <% if (hasSuggestions) { %>MESSAGE_ID_ERROR<% } else { %>MESSAGE_ID<% } %>,

diff --git a/test/consistent-empty-array-spread.mjs b/test/consistent-empty-array-spread.mjs
@@ -0,0 +1,57 @@
+import outdent from 'outdent';
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'[,,,]',
+		'[...(test ? [] : [a, b])]',
+		'[...(test ? [a, b] : [])]',
+		'[...(test ? "" : "ab")]',
+		'[...(test ? "ab" : "")]',
+		'[...(test ? "" : unknown)]',
+		'[...(test ? unknown : "")]',
+		'[...(test ? [] : unknown)]',
+		'[...(test ? unknown : [])]',
+		'_ = {...(test ? "" : [a, b])}',
+		'_ = {...(test ? [] : "ab")}',
+		'call(...(test ? "" : [a, b]))',
+		'call(...(test ? [] : "ab"))',
+		'[...(test ? "ab" : [a, b])]',
+		// Not checking
+		'const EMPTY_STRING = ""; [...(test ? EMPTY_STRING : [a, b])]',
+	],
+	invalid: [
+		outdent`
+			[
+				...(test ? [] : "ab"),
+				...(test ? "ab" : []),
+			];
+		`,
+		outdent`
+			const STRING = "ab";
+			[
+				...(test ? [] : STRING),
+				...(test ? STRING : []),
+			];
+		`,
+		outdent`
+			[
+				...(test ? "" : [a, b]),
+				...(test ? [a, b] : ""),
+			];
+		`,
+		outdent`
+			const ARRAY = ["a", "b"];
+			[
+				/* hole */,
+				...(test ? "" : ARRAY),
+				/* hole */,
+				...(test ? ARRAY : ""),
+				/* hole */,
+			];
+		`,
+		'[...(foo ? "" : [])]',
+	],
+});