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Add consistent-empty-array-spread rule (#2349)
Co-authored-by: Sindre Sorhus <[email protected]>
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‎docs/rules/consistent-empty-array-spread.md

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# Prefer consistent types when spreading a ternary in an array literal
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💼 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs-eslintconfigjs).
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🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
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<!-- end auto-generated rule header -->
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<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
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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.
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## Fail
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```js
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const array = [
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a,
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...(foo ? [b, c] : ''),
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];
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```
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```js
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const array = [
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a,
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...(foo ? 'bc' : []),
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];
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```
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## Pass
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```js
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const array = [
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a,
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...(foo ? [b, c] : []),
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];
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```
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```js
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const array = [
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a,
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...(foo ? 'bc' : ''),
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];
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```

‎readme.md

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@@ -113,6 +113,7 @@ If you don't use the preset, ensure you use the same `env` and `parserOptions` c
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| [better-regex](docs/rules/better-regex.md) | Improve regexes by making them shorter, consistent, and safer. | ✅ | 🔧 | |
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| [catch-error-name](docs/rules/catch-error-name.md) | Enforce a specific parameter name in catch clauses. | ✅ | 🔧 | |
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| [consistent-destructuring](docs/rules/consistent-destructuring.md) | Use destructured variables over properties. | | 🔧 | 💡 |
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| [consistent-empty-array-spread](docs/rules/consistent-empty-array-spread.md) | Prefer consistent types when spreading a ternary in an array literal. | ✅ | 🔧 | |
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| [consistent-function-scoping](docs/rules/consistent-function-scoping.md) | Move function definitions to the highest possible scope. | ✅ | | |
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| [custom-error-definition](docs/rules/custom-error-definition.md) | Enforce correct `Error` subclassing. | | 🔧 | |
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| [empty-brace-spaces](docs/rules/empty-brace-spaces.md) | Enforce no spaces between braces. | ✅ | 🔧 | |

‎rules/consistent-empty-array-spread.js

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

‎scripts/template/rule.js.jst

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};
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<% } %>
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const selector = [
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'Literal',
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'[value="unicorn"]',
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].join('');
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/** @param {import('eslint').Rule.RuleContext} context */
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const create = context => {
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return {
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[selector](node) {
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Literal(node) {
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if (node.value !== 'unicorn') {
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return;
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}
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return {
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node,
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messageId: <% if (hasSuggestions) { %>MESSAGE_ID_ERROR<% } else { %>MESSAGE_ID<% } %>,

‎test/consistent-empty-array-spread.mjs

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import outdent from 'outdent';
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import {getTester} from './utils/test.mjs';
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const {test} = getTester(import.meta);
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test.snapshot({
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valid: [
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'[,,,]',
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'[...(test ? [] : [a, b])]',
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'[...(test ? [a, b] : [])]',
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'[...(test ? "" : "ab")]',
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'[...(test ? "ab" : "")]',
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'[...(test ? "" : unknown)]',
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'[...(test ? unknown : "")]',
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'[...(test ? [] : unknown)]',
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'[...(test ? unknown : [])]',
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'_ = {...(test ? "" : [a, b])}',
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'_ = {...(test ? [] : "ab")}',
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'call(...(test ? "" : [a, b]))',
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'call(...(test ? [] : "ab"))',
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'[...(test ? "ab" : [a, b])]',
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// Not checking
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'const EMPTY_STRING = ""; [...(test ? EMPTY_STRING : [a, b])]',
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],
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invalid: [
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outdent`
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[
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...(test ? [] : "ab"),
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...(test ? "ab" : []),
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];
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`,
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outdent`
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const STRING = "ab";
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[
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...(test ? [] : STRING),
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...(test ? STRING : []),
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];
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`,
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outdent`
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[
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...(test ? "" : [a, b]),
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...(test ? [a, b] : ""),
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];
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`,
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outdent`
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const ARRAY = ["a", "b"];
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[
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/* hole */,
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...(test ? "" : ARRAY),
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/* hole */,
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...(test ? ARRAY : ""),
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/* hole */,
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];
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`,
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'[...(foo ? "" : [])]',
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],
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});

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