Add no-useless-fallback-in-spread rule (#1481)

Author: fisker

docs/rules/no-useless-fallback-in-spread.md

@@ -0,0 +1,29 @@
+# Forbid useless fallback when spreading in object literals
+
+Spreading [falsy values](https://developer.mozilla.org/en-US/docs/Glossary/Falsy) in object literals won't add any unexpected properties, so it's unnecessary to add an empty object as fallback.
+
+This rule is fixable.
+
+## Fail
+
+```js
+const object = {...(foo || {})};
+```
+
+```js
+const object = {...(foo ?? {})};
+```
+
+## Pass
+
+```js
+const object = {...foo};
+```
+
+```js
+const object = {...(foo && {})};
+```
+
+```js
+const array = [...(foo || [])];
+```

index.js

@@ -78,6 +78,7 @@ module.exports = {
 				'unicorn/no-unreadable-array-destructuring': 'error',
 				'unicorn/no-unsafe-regex': 'off',
 				'unicorn/no-unused-properties': 'off',
+				'unicorn/no-useless-fallback-in-spread': 'error',
 				'unicorn/no-useless-length-check': 'error',
 				'unicorn/no-useless-spread': 'error',
 				'unicorn/no-useless-undefined': 'error',

readme.md

@@ -75,6 +75,7 @@ Configure it in `package.json`.
 			"unicorn/no-unreadable-array-destructuring": "error",
 			"unicorn/no-unsafe-regex": "off",
 			"unicorn/no-unused-properties": "off",
+			"unicorn/no-useless-fallback-in-spread": "error",
 			"unicorn/no-useless-length-check": "error",
 			"unicorn/no-useless-spread": "error",
 			"unicorn/no-useless-undefined": "error",
@@ -194,6 +195,7 @@ Each rule has emojis denoting:
 | [no-unreadable-array-destructuring](docs/rules/no-unreadable-array-destructuring.md) | Disallow unreadable array destructuring. | ✅ | 🔧 |  |
 | [no-unsafe-regex](docs/rules/no-unsafe-regex.md) | Disallow unsafe regular expressions. |  |  |  |
 | [no-unused-properties](docs/rules/no-unused-properties.md) | Disallow unused object properties. |  |  |  |
+| [no-useless-fallback-in-spread](docs/rules/no-useless-fallback-in-spread.md) | Forbid useless fallback when spreading in object literals. | ✅ | 🔧 |  |
 | [no-useless-length-check](docs/rules/no-useless-length-check.md) | Disallow useless array length check. | ✅ | 🔧 |  |
 | [no-useless-spread](docs/rules/no-useless-spread.md) | Disallow unnecessary spread. | ✅ | 🔧 |  |
 | [no-useless-undefined](docs/rules/no-useless-undefined.md) | Disallow useless `undefined`. | ✅ | 🔧 |  |

rules/no-useless-fallback-in-spread.js

@@ -0,0 +1,76 @@
+'use strict';
+const {matches} = require('./selectors/index.js');
+const {
+	isParenthesized,
+	getParenthesizedRange,
+	getParentheses,
+} = require('./utils/parentheses.js');
+const shouldAddParenthesesToSpreadElementArgument = require('./utils/should-add-parentheses-to-spread-element-argument.js');
+
+const MESSAGE_ID = 'no-useless-fallback-in-spread';
+const messages = {
+	[MESSAGE_ID]: 'The empty object is useless.',
+};
+
+const selector = [
+	'ObjectExpression',
+	' > ',
+	'SpreadElement.properties',
+	' > ',
+	'LogicalExpression.argument',
+	matches([
+		'[operator="||"]',
+		'[operator="??"]',
+	]),
+	' > ',
+	'ObjectExpression[properties.length=0].right',
+].join('');
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => ({
+	[selector](emptyObject) {
+		return {
+			node: emptyObject,
+			messageId: MESSAGE_ID,
+			/** @param {import('eslint').Rule.RuleFixer} fixer */
+			* fix(fixer) {
+				const sourceCode = context.getSourceCode();
+				const logicalExpression = emptyObject.parent;
+				const {left} = logicalExpression;
+				const isLeftObjectParenthesized = isParenthesized(left, sourceCode);
+				const [, start] = isLeftObjectParenthesized
+					? getParenthesizedRange(left, sourceCode)
+					: left.range;
+				const [, end] = logicalExpression.range;
+
+				yield fixer.removeRange([start, end]);
+
+				if (
+					isLeftObjectParenthesized
+					|| !shouldAddParenthesesToSpreadElementArgument(left)
+				) {
+					const parentheses = getParentheses(logicalExpression, sourceCode);
+
+					for (const token of parentheses) {
+						yield fixer.remove(token);
+					}
+				}
+			},
+		};
+	},
+});
+
+const schema = [];
+
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Forbid useless fallback when spreading in object literals.',
+		},
+		fixable: 'code',
+		schema,
+		messages,
+	},
+};

test/no-useless-fallback-in-spread.mjs

@@ -0,0 +1,53 @@
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'const array = [...(foo || [])]',
+		'const array = [...(foo || {})]',
+		'const array = [...(foo && {})]',
+		'const object = {...(foo && {})}',
+		'const object = {...({} || foo)}',
+		'const object = {...({} && foo)}',
+		'const object = {...({} ?? foo)}',
+		'const object = {...(foo ? foo : {})}',
+		'const object = {...foo}',
+		'const object = {...(foo ?? ({} || {}))}',
+		'const {...foo} = object',
+		'function foo({...bar}){}',
+		'const object = {...(foo || {}).toString()}',
+		'const object = {...fn(foo || {})}',
+		'const object = call({}, ...(foo || {}))',
+		'const object = {...(foo || {not: "empty"})}',
+		'const object = {...(foo || {...{}})}',
+	],
+	invalid: [
+		'const object = {...(foo || {})}',
+		'const object = {...(foo ?? {})}',
+		'const object = {...(foo ?? (( {} )))}',
+		'const object = {...((( foo )) ?? (( {} )))}',
+		'const object = {...(( (( foo )) ?? (( {} )) ))}',
+		'async ()=> ({...((await foo) || {})})',
+		'const object = {...(0 || {})}',
+		'const object = {...((-0) || {})}',
+		'const object = {...(.0 || {})}',
+		'const object = {...(0n || {})}',
+		'const object = {...(false || {})}',
+		'const object = {...(null || {})}',
+		'const object = {...(undefined || {})}',
+		'const object = {...((a && b) || {})}',
+		'const object = {...(NaN || {})}',
+		'const object = {...("" || {})}',
+		'const object = {...([] || {})}',
+		'const object = {...({} || {})}',
+		'const object = {...(foo || {}),}',
+		'const object = {...((foo ?? {}) || {})}',
+		'const object = {...((foo && {}) || {})}',
+		'const object = {...(foo && {} || {})}',
+		'const object = {...({...(foo || {})})}',
+		'function foo(a = {...(bar || {})}){}',
+		// The only case we'll break, but we should not care about it.
+		'const object = {...(document.all || {})}',
+	],
+});