From eba5d52a89fadbf9b4c9dc26786bc2884171993d Mon Sep 17 00:00:00 2001
From: Srijan Saurav <68371686+srijan-deepsource@users.noreply.github.com>
Date: Mon, 30 May 2022 16:19:26 +0530
Subject: [PATCH] Revert "chore: enable some rules from eslint-plugin-unicorn
 internally (#15878)"

This reverts commit bfa354597066e20d32dc3a38fe4b544f323b861d.
---
 Makefile.js                                       |  6 +++---
 lib/cli-engine/cli-engine.js                      |  6 ++++--
 lib/cli-engine/lint-result-cache.js               |  2 +-
 .../code-path-analysis/code-path-segment.js       |  2 +-
 lib/linter/code-path-analysis/code-path-state.js  |  2 +-
 lib/linter/code-path-analysis/code-path.js        |  2 +-
 lib/rules/accessor-pairs.js                       |  8 ++++----
 lib/rules/callback-return.js                      |  4 ++--
 lib/rules/capitalized-comments.js                 |  2 +-
 lib/rules/consistent-this.js                      |  2 +-
 lib/rules/dot-notation.js                         |  4 ++--
 lib/rules/global-require.js                       |  6 +++---
 lib/rules/indent-legacy.js                        |  4 ++--
 lib/rules/indent.js                               |  2 +-
 lib/rules/jsx-quotes.js                           |  2 +-
 lib/rules/lines-around-comment.js                 |  6 +++---
 lib/rules/max-lines.js                            |  4 ++--
 lib/rules/newline-before-return.js                |  2 +-
 lib/rules/no-bitwise.js                           |  4 ++--
 lib/rules/no-console.js                           |  2 +-
 lib/rules/no-empty-function.js                    |  2 +-
 lib/rules/no-extra-boolean-cast.js                |  6 +++---
 lib/rules/no-extra-semi.js                        |  2 +-
 lib/rules/no-global-assign.js                     |  2 +-
 lib/rules/no-implicit-coercion.js                 | 12 ++++++------
 lib/rules/no-magic-numbers.js                     |  6 +++---
 lib/rules/no-mixed-operators.js                   |  2 +-
 lib/rules/no-mixed-requires.js                    |  2 +-
 lib/rules/no-multi-spaces.js                      |  2 +-
 lib/rules/no-native-reassign.js                   |  2 +-
 lib/rules/no-new-wrappers.js                      |  2 +-
 lib/rules/no-prototype-builtins.js                |  6 +++---
 lib/rules/no-shadow.js                            | 10 +++++-----
 lib/rules/no-sparse-arrays.js                     |  2 +-
 lib/rules/no-underscore-dangle.js                 |  2 +-
 lib/rules/no-unused-expressions.js                |  2 +-
 lib/rules/no-unused-vars.js                       |  2 +-
 lib/rules/operator-assignment.js                  |  4 ++--
 lib/rules/prefer-const.js                         |  2 +-
 lib/rules/prefer-reflect.js                       |  4 ++--
 lib/rules/prefer-regex-literals.js                |  6 +++---
 lib/rules/quote-props.js                          |  4 ++--
 lib/rules/quotes.js                               |  2 +-
 lib/rules/spaced-comment.js                       |  2 +-
 lib/rules/valid-jsdoc.js                          |  2 +-
 lib/rules/valid-typeof.js                         |  8 ++++----
 lib/rules/yoda.js                                 |  2 +-
 package.json                                      |  1 -
 packages/eslint-config-eslint/default.yml         | 15 ---------------
 packages/eslint-config-eslint/package.json        |  3 +--
 tests/lib/cli.js                                  |  6 +++---
 tests/lib/linter/node-event-generator.js          |  4 ++--
 tests/lib/rules/no-invalid-this.js                |  3 ++-
 tools/code-sample-minimizer.js                    |  6 ++++--
 54 files changed, 99 insertions(+), 111 deletions(-)

diff --git a/Makefile.js b/Makefile.js
index 49d3093c1f0..e0a19541468 100644
--- a/Makefile.js
+++ b/Makefile.js
@@ -614,7 +614,7 @@ target.gensite = function(prereleaseVersion) {
         if (test("-f", fullPath)) {
             rm("-rf", fullPath);
 
-            if (filePath.includes(".md") && test("-f", htmlFullPath)) {
+            if (filePath.indexOf(".md") >= 0 && test("-f", htmlFullPath)) {
                 rm("-rf", htmlFullPath);
             }
         }
@@ -673,7 +673,7 @@ target.gensite = function(prereleaseVersion) {
             process.stdout.write(`> Updating files (Steps 4-9): ${i}/${length} - ${filePath + " ".repeat(30)}\r`);
 
             // 5. Prepend page title and layout variables at the top of rules
-            if (path.dirname(filename).includes("rules")) {
+            if (path.dirname(filename).indexOf("rules") >= 0) {
 
                 // Find out if the rule requires a special docs portion (e.g. if it is recommended and/or fixable)
                 const rule = rules.get(ruleName);
@@ -696,7 +696,7 @@ target.gensite = function(prereleaseVersion) {
             }
 
             // 8. Append first version of ESLint rule was added at.
-            if (filename.includes("rules/")) {
+            if (filename.indexOf("rules/") !== -1) {
                 if (!versions.added[baseName]) {
                     versions.added[baseName] = getFirstVersionOfFile(sourcePath);
                 }
diff --git a/lib/cli-engine/cli-engine.js b/lib/cli-engine/cli-engine.js
index fdc66198809..f7aa9e2242a 100644
--- a/lib/cli-engine/cli-engine.js
+++ b/lib/cli-engine/cli-engine.js
@@ -366,7 +366,9 @@ function *iterateRuleDeprecationWarnings(usedConfigArrays) {
 
     // Flatten used configs.
     /** @type {ExtractedConfig[]} */
-    const configs = usedConfigArrays.flatMap(getUsedExtractedConfigs);
+    const configs = [].concat(
+        ...usedConfigArrays.map(getUsedExtractedConfigs)
+    );
 
     // Traverse rule configs.
     for (const config of configs) {
@@ -1021,7 +1023,7 @@ class CLIEngine {
             let formatterPath;
 
             // if there's a slash, then it's a file (TODO: this check seems dubious for scoped npm packages)
-            if (!namespace && normalizedFormatName.includes("/")) {
+            if (!namespace && normalizedFormatName.indexOf("/") > -1) {
                 formatterPath = path.resolve(cwd, normalizedFormatName);
             } else {
                 try {
diff --git a/lib/cli-engine/lint-result-cache.js b/lib/cli-engine/lint-result-cache.js
index e36eb74bada..3304d429deb 100644
--- a/lib/cli-engine/lint-result-cache.js
+++ b/lib/cli-engine/lint-result-cache.js
@@ -36,7 +36,7 @@ const invalidCacheStrategyErrorMessage = `Cache strategy must be one of: ${valid
  */
 function isValidCacheStrategy(cacheStrategy) {
     return (
-        validCacheStrategies.includes(cacheStrategy)
+        validCacheStrategies.indexOf(cacheStrategy) !== -1
     );
 }
 
diff --git a/lib/linter/code-path-analysis/code-path-segment.js b/lib/linter/code-path-analysis/code-path-segment.js
index eeedd27524b..fad559a2ad2 100644
--- a/lib/linter/code-path-analysis/code-path-segment.js
+++ b/lib/linter/code-path-analysis/code-path-segment.js
@@ -100,7 +100,7 @@ class CodePathSegment {
      * @returns {boolean} `true` if the segment is coming from the end of a loop.
      */
     isLoopedPrevSegment(segment) {
-        return this.internal.loopedPrevSegments.includes(segment);
+        return this.internal.loopedPrevSegments.indexOf(segment) !== -1;
     }
 
     /**
diff --git a/lib/linter/code-path-analysis/code-path-state.js b/lib/linter/code-path-analysis/code-path-state.js
index 426da8d492e..118f70a1707 100644
--- a/lib/linter/code-path-analysis/code-path-state.js
+++ b/lib/linter/code-path-analysis/code-path-state.js
@@ -33,7 +33,7 @@ function addToReturnedOrThrown(dest, others, all, segments) {
         const segment = segments[i];
 
         dest.push(segment);
-        if (!others.includes(segment)) {
+        if (others.indexOf(segment) === -1) {
             all.push(segment);
         }
     }
diff --git a/lib/linter/code-path-analysis/code-path.js b/lib/linter/code-path-analysis/code-path.js
index a028ca69481..0e666277094 100644
--- a/lib/linter/code-path-analysis/code-path.js
+++ b/lib/linter/code-path-analysis/code-path.js
@@ -212,7 +212,7 @@ class CodePath {
                 }
 
                 // Reset the flag of skipping if all branches have been skipped.
-                if (skippedSegment && segment.prevSegments.includes(skippedSegment)) {
+                if (skippedSegment && segment.prevSegments.indexOf(skippedSegment) !== -1) {
                     skippedSegment = null;
                 }
                 visited[segment.id] = true;
diff --git a/lib/rules/accessor-pairs.js b/lib/rules/accessor-pairs.js
index 33887affef2..2fcf1535290 100644
--- a/lib/rules/accessor-pairs.js
+++ b/lib/rules/accessor-pairs.js
@@ -299,12 +299,12 @@ module.exports = {
          * @private
          */
         function checkPropertyDescriptor(node) {
-            const namesToCheck = new Set(node.properties
+            const namesToCheck = node.properties
                 .filter(p => p.type === "Property" && p.kind === "init" && !p.computed)
-                .map(({ key }) => key.name));
+                .map(({ key }) => key.name);
 
-            const hasGetter = namesToCheck.has("get");
-            const hasSetter = namesToCheck.has("set");
+            const hasGetter = namesToCheck.includes("get");
+            const hasSetter = namesToCheck.includes("set");
 
             if (checkSetWithoutGet && hasSetter && !hasGetter) {
                 report(node, "missingGetter");
diff --git a/lib/rules/callback-return.js b/lib/rules/callback-return.js
index 34b74631f0c..532faee4518 100644
--- a/lib/rules/callback-return.js
+++ b/lib/rules/callback-return.js
@@ -53,7 +53,7 @@ module.exports = {
             if (!node.parent) {
                 return null;
             }
-            if (!types.includes(node.parent.type)) {
+            if (types.indexOf(node.parent.type) === -1) {
                 return findClosestParentOfType(node.parent, types);
             }
             return node.parent;
@@ -87,7 +87,7 @@ module.exports = {
          * @returns {boolean} Whether or not this function matches our callback name.
          */
         function isCallback(node) {
-            return containsOnlyIdentifiers(node.callee) && callbacks.includes(sourceCode.getText(node.callee));
+            return containsOnlyIdentifiers(node.callee) && callbacks.indexOf(sourceCode.getText(node.callee)) > -1;
         }
 
         /**
diff --git a/lib/rules/capitalized-comments.js b/lib/rules/capitalized-comments.js
index dffe9565003..6ea87a70a81 100644
--- a/lib/rules/capitalized-comments.js
+++ b/lib/rules/capitalized-comments.js
@@ -185,7 +185,7 @@ module.exports = {
 
             return Boolean(
                 previousTokenOrComment &&
-                ["Block", "Line"].includes(previousTokenOrComment.type)
+                ["Block", "Line"].indexOf(previousTokenOrComment.type) !== -1
             );
         }
 
diff --git a/lib/rules/consistent-this.js b/lib/rules/consistent-this.js
index 2aa07914638..a613f992d7a 100644
--- a/lib/rules/consistent-this.js
+++ b/lib/rules/consistent-this.js
@@ -65,7 +65,7 @@ module.exports = {
         function checkAssignment(node, name, value) {
             const isThis = value.type === "ThisExpression";
 
-            if (aliases.includes(name)) {
+            if (aliases.indexOf(name) !== -1) {
                 if (!isThis || node.operator && node.operator !== "=") {
                     reportBadAssignment(node, name);
                 }
diff --git a/lib/rules/dot-notation.js b/lib/rules/dot-notation.js
index 90ba6d83b76..6f6b5166f76 100644
--- a/lib/rules/dot-notation.js
+++ b/lib/rules/dot-notation.js
@@ -76,7 +76,7 @@ module.exports = {
         function checkComputedProperty(node, value) {
             if (
                 validIdentifier.test(value) &&
-                (allowKeywords || !keywords.includes(String(value))) &&
+                (allowKeywords || keywords.indexOf(String(value)) === -1) &&
                 !(allowPattern && allowPattern.test(value))
             ) {
                 const formattedValue = node.property.type === "Literal" ? JSON.stringify(value) : `\`${value}\``;
@@ -142,7 +142,7 @@ module.exports = {
                     !allowKeywords &&
                     !node.computed &&
                     node.property.type === "Identifier" &&
-                    keywords.includes(String(node.property.name))
+                    keywords.indexOf(String(node.property.name)) !== -1
                 ) {
                     context.report({
                         node: node.property,
diff --git a/lib/rules/global-require.js b/lib/rules/global-require.js
index 71a2bf1ac49..f00ca9bf6dd 100644
--- a/lib/rules/global-require.js
+++ b/lib/rules/global-require.js
@@ -6,7 +6,7 @@
 
 "use strict";
 
-const ACCEPTABLE_PARENTS = new Set([
+const ACCEPTABLE_PARENTS = [
     "AssignmentExpression",
     "VariableDeclarator",
     "MemberExpression",
@@ -16,7 +16,7 @@ const ACCEPTABLE_PARENTS = new Set([
     "Program",
     "VariableDeclaration",
     "ChainExpression"
-]);
+];
 
 /**
  * Finds the eslint-scope reference in the given scope.
@@ -75,7 +75,7 @@ module.exports = {
                 const currentScope = context.getScope();
 
                 if (node.callee.name === "require" && !isShadowed(currentScope, node.callee)) {
-                    const isGoodRequire = context.getAncestors().every(parent => ACCEPTABLE_PARENTS.has(parent.type));
+                    const isGoodRequire = context.getAncestors().every(parent => ACCEPTABLE_PARENTS.indexOf(parent.type) > -1);
 
                     if (!isGoodRequire) {
                         context.report({ node, messageId: "unexpected" });
diff --git a/lib/rules/indent-legacy.js b/lib/rules/indent-legacy.js
index c5be4a5d134..bd61f6f8431 100644
--- a/lib/rules/indent-legacy.js
+++ b/lib/rules/indent-legacy.js
@@ -753,7 +753,7 @@ module.exports = {
                             if (typeof options.CallExpression.arguments === "number") {
                                 nodeIndent += options.CallExpression.arguments * indentSize;
                             } else if (options.CallExpression.arguments === "first") {
-                                if (parent.arguments.includes(node)) {
+                                if (parent.arguments.indexOf(node) !== -1) {
                                     nodeIndent = parent.arguments[0].loc.start.column;
                                 }
                             } else {
@@ -840,7 +840,7 @@ module.exports = {
                 "IfStatement", "WhileStatement", "ForStatement", "ForInStatement", "ForOfStatement", "DoWhileStatement", "ClassDeclaration", "TryStatement"
             ];
 
-            if (node.parent && statementsWithProperties.includes(node.parent.type) && isNodeBodyBlock(node)) {
+            if (node.parent && statementsWithProperties.indexOf(node.parent.type) !== -1 && isNodeBodyBlock(node)) {
                 indent = getNodeIndent(node.parent).goodChar;
             } else if (node.parent && node.parent.type === "CatchClause") {
                 indent = getNodeIndent(node.parent.parent).goodChar;
diff --git a/lib/rules/indent.js b/lib/rules/indent.js
index 9c534cd58f0..6152bc561f9 100644
--- a/lib/rules/indent.js
+++ b/lib/rules/indent.js
@@ -796,7 +796,7 @@ module.exports = {
             let statement = node.parent && node.parent.parent;
 
             while (
-                statement.type === "UnaryExpression" && ["!", "~", "+", "-"].includes(statement.operator) ||
+                statement.type === "UnaryExpression" && ["!", "~", "+", "-"].indexOf(statement.operator) > -1 ||
                 statement.type === "AssignmentExpression" ||
                 statement.type === "LogicalExpression" ||
                 statement.type === "SequenceExpression" ||
diff --git a/lib/rules/jsx-quotes.js b/lib/rules/jsx-quotes.js
index f63dfd608bc..a0cdfaa62be 100644
--- a/lib/rules/jsx-quotes.js
+++ b/lib/rules/jsx-quotes.js
@@ -70,7 +70,7 @@ module.exports = {
          * @public
          */
         function usesExpectedQuotes(node) {
-            return node.value.includes(setting.quote) || astUtils.isSurroundedBy(node.raw, setting.quote);
+            return node.value.indexOf(setting.quote) !== -1 || astUtils.isSurroundedBy(node.raw, setting.quote);
         }
 
         return {
diff --git a/lib/rules/lines-around-comment.js b/lib/rules/lines-around-comment.js
index 6b1cd5f848b..74df09b01bd 100644
--- a/lib/rules/lines-around-comment.js
+++ b/lib/rules/lines-around-comment.js
@@ -141,7 +141,7 @@ module.exports = {
             comments = sourceCode.getAllComments(),
             commentLines = getCommentLineNums(comments),
             emptyLines = getEmptyLineNums(lines),
-            commentAndEmptyLines = new Set(commentLines.concat(emptyLines));
+            commentAndEmptyLines = commentLines.concat(emptyLines);
 
         /**
          * Returns whether or not comments are on lines starting with or ending with code
@@ -393,7 +393,7 @@ module.exports = {
             const nextTokenOrComment = sourceCode.getTokenAfter(token, { includeComments: true });
 
             // check for newline before
-            if (!exceptionStartAllowed && before && !commentAndEmptyLines.has(prevLineNum) &&
+            if (!exceptionStartAllowed && before && !commentAndEmptyLines.includes(prevLineNum) &&
                     !(astUtils.isCommentToken(previousTokenOrComment) && astUtils.isTokenOnSameLine(previousTokenOrComment, token))) {
                 const lineStart = token.range[0] - token.loc.start.column;
                 const range = [lineStart, lineStart];
@@ -408,7 +408,7 @@ module.exports = {
             }
 
             // check for newline after
-            if (!exceptionEndAllowed && after && !commentAndEmptyLines.has(nextLineNum) &&
+            if (!exceptionEndAllowed && after && !commentAndEmptyLines.includes(nextLineNum) &&
                     !(astUtils.isCommentToken(nextTokenOrComment) && astUtils.isTokenOnSameLine(token, nextTokenOrComment))) {
                 context.report({
                     node: token,
diff --git a/lib/rules/max-lines.js b/lib/rules/max-lines.js
index 772f02aff08..31fc657a63c 100644
--- a/lib/rules/max-lines.js
+++ b/lib/rules/max-lines.js
@@ -159,10 +159,10 @@ module.exports = {
                 if (skipComments) {
                     const comments = sourceCode.getAllComments();
 
-                    const commentLines = new Set(comments.flatMap(getLinesWithoutCode));
+                    const commentLines = comments.flatMap(getLinesWithoutCode);
 
                     lines = lines.filter(
-                        l => !commentLines.has(l.lineNumber)
+                        l => !commentLines.includes(l.lineNumber)
                     );
                 }
 
diff --git a/lib/rules/newline-before-return.js b/lib/rules/newline-before-return.js
index d07c23658ed..f4aa5166f0e 100644
--- a/lib/rules/newline-before-return.js
+++ b/lib/rules/newline-before-return.js
@@ -47,7 +47,7 @@ module.exports = {
         function isPrecededByTokens(node, testTokens) {
             const tokenBefore = sourceCode.getTokenBefore(node);
 
-            return testTokens.includes(tokenBefore.value);
+            return testTokens.some(token => tokenBefore.value === token);
         }
 
         /**
diff --git a/lib/rules/no-bitwise.js b/lib/rules/no-bitwise.js
index 43a1e764edc..1de1619bffa 100644
--- a/lib/rules/no-bitwise.js
+++ b/lib/rules/no-bitwise.js
@@ -76,7 +76,7 @@ module.exports = {
          * @returns {boolean} Whether or not the node has a bitwise operator.
          */
         function hasBitwiseOperator(node) {
-            return BITWISE_OPERATORS.includes(node.operator);
+            return BITWISE_OPERATORS.indexOf(node.operator) !== -1;
         }
 
         /**
@@ -85,7 +85,7 @@ module.exports = {
          * @returns {boolean} Whether or not the node has a bitwise operator.
          */
         function allowedOperator(node) {
-            return allowed.includes(node.operator);
+            return allowed.indexOf(node.operator) !== -1;
         }
 
         /**
diff --git a/lib/rules/no-console.js b/lib/rules/no-console.js
index 464d5647cc3..c0802e23dc7 100644
--- a/lib/rules/no-console.js
+++ b/lib/rules/no-console.js
@@ -72,7 +72,7 @@ module.exports = {
         function isAllowed(node) {
             const propertyName = astUtils.getStaticPropertyName(node);
 
-            return propertyName && allowed.includes(propertyName);
+            return propertyName && allowed.indexOf(propertyName) !== -1;
         }
 
         /**
diff --git a/lib/rules/no-empty-function.js b/lib/rules/no-empty-function.js
index e23b69e537b..6e73d63b98a 100644
--- a/lib/rules/no-empty-function.js
+++ b/lib/rules/no-empty-function.js
@@ -144,7 +144,7 @@ module.exports = {
                 filter: astUtils.isCommentToken
             });
 
-            if (!allowed.includes(kind) &&
+            if (allowed.indexOf(kind) === -1 &&
                 node.body.type === "BlockStatement" &&
                 node.body.body.length === 0 &&
                 innerComments.length === 0
diff --git a/lib/rules/no-extra-boolean-cast.js b/lib/rules/no-extra-boolean-cast.js
index ddb1d8330d5..f45ac616ed9 100644
--- a/lib/rules/no-extra-boolean-cast.js
+++ b/lib/rules/no-extra-boolean-cast.js
@@ -51,13 +51,13 @@ module.exports = {
         const sourceCode = context.getSourceCode();
 
         // Node types which have a test which will coerce values to booleans.
-        const BOOLEAN_NODE_TYPES = new Set([
+        const BOOLEAN_NODE_TYPES = [
             "IfStatement",
             "DoWhileStatement",
             "WhileStatement",
             "ConditionalExpression",
             "ForStatement"
-        ]);
+        ];
 
         /**
          * Check if a node is a Boolean function or constructor.
@@ -95,7 +95,7 @@ module.exports = {
                 (isBooleanFunctionOrConstructorCall(node.parent) &&
                 node === node.parent.arguments[0]) ||
 
-                (BOOLEAN_NODE_TYPES.has(node.parent.type) &&
+                (BOOLEAN_NODE_TYPES.indexOf(node.parent.type) !== -1 &&
                     node === node.parent.test) ||
 
                 // !<bool>
diff --git a/lib/rules/no-extra-semi.js b/lib/rules/no-extra-semi.js
index 625310eea69..89d9fce57f0 100644
--- a/lib/rules/no-extra-semi.js
+++ b/lib/rules/no-extra-semi.js
@@ -98,7 +98,7 @@ module.exports = {
                         "WithStatement"
                     ];
 
-                if (!allowedParentTypes.includes(parent.type)) {
+                if (allowedParentTypes.indexOf(parent.type) === -1) {
                     report(node);
                 }
             },
diff --git a/lib/rules/no-global-assign.js b/lib/rules/no-global-assign.js
index 1225baec5de..340c6c798c3 100644
--- a/lib/rules/no-global-assign.js
+++ b/lib/rules/no-global-assign.js
@@ -78,7 +78,7 @@ module.exports = {
          * @returns {void}
          */
         function checkVariable(variable) {
-            if (variable.writeable === false && !exceptions.includes(variable.name)) {
+            if (variable.writeable === false && exceptions.indexOf(variable.name) === -1) {
                 variable.references.forEach(checkReference);
             }
         }
diff --git a/lib/rules/no-implicit-coercion.js b/lib/rules/no-implicit-coercion.js
index 4b21e3d94f4..f646093f555 100644
--- a/lib/rules/no-implicit-coercion.js
+++ b/lib/rules/no-implicit-coercion.js
@@ -257,7 +257,7 @@ module.exports = {
                 let operatorAllowed;
 
                 // !!foo
-                operatorAllowed = options.allow.includes("!!");
+                operatorAllowed = options.allow.indexOf("!!") >= 0;
                 if (!operatorAllowed && options.boolean && isDoubleLogicalNegating(node)) {
                     const recommendation = `Boolean(${sourceCode.getText(node.argument.argument)})`;
 
@@ -265,7 +265,7 @@ module.exports = {
                 }
 
                 // ~foo.indexOf(bar)
-                operatorAllowed = options.allow.includes("~");
+                operatorAllowed = options.allow.indexOf("~") >= 0;
                 if (!operatorAllowed && options.boolean && isBinaryNegatingOfIndexOf(node)) {
 
                     // `foo?.indexOf(bar) !== -1` will be true (== found) if the `foo` is nullish. So use `>= 0` in that case.
@@ -276,7 +276,7 @@ module.exports = {
                 }
 
                 // +foo
-                operatorAllowed = options.allow.includes("+");
+                operatorAllowed = options.allow.indexOf("+") >= 0;
                 if (!operatorAllowed && options.number && node.operator === "+" && !isNumeric(node.argument)) {
                     const recommendation = `Number(${sourceCode.getText(node.argument)})`;
 
@@ -289,7 +289,7 @@ module.exports = {
                 let operatorAllowed;
 
                 // 1 * foo
-                operatorAllowed = options.allow.includes("*");
+                operatorAllowed = options.allow.indexOf("*") >= 0;
                 const nonNumericOperand = !operatorAllowed && options.number && isMultiplyByOne(node) && getNonNumericOperand(node);
 
                 if (nonNumericOperand) {
@@ -299,7 +299,7 @@ module.exports = {
                 }
 
                 // "" + foo
-                operatorAllowed = options.allow.includes("+");
+                operatorAllowed = options.allow.indexOf("+") >= 0;
                 if (!operatorAllowed && options.string && isConcatWithEmptyString(node)) {
                     const recommendation = `String(${sourceCode.getText(getNonEmptyOperand(node))})`;
 
@@ -310,7 +310,7 @@ module.exports = {
             AssignmentExpression(node) {
 
                 // foo += ""
-                const operatorAllowed = options.allow.includes("+");
+                const operatorAllowed = options.allow.indexOf("+") >= 0;
 
                 if (!operatorAllowed && options.string && isAppendEmptyString(node)) {
                     const code = sourceCode.getText(getNonEmptyOperand(node));
diff --git a/lib/rules/no-magic-numbers.js b/lib/rules/no-magic-numbers.js
index 6b4cf77c326..dfcd593ecbe 100644
--- a/lib/rules/no-magic-numbers.js
+++ b/lib/rules/no-magic-numbers.js
@@ -80,7 +80,7 @@ module.exports = {
         const config = context.options[0] || {},
             detectObjects = !!config.detectObjects,
             enforceConst = !!config.enforceConst,
-            ignore = new Set((config.ignore || []).map(normalizeIgnoreValue)),
+            ignore = (config.ignore || []).map(normalizeIgnoreValue),
             ignoreArrayIndexes = !!config.ignoreArrayIndexes,
             ignoreDefaultValues = !!config.ignoreDefaultValues;
 
@@ -92,7 +92,7 @@ module.exports = {
          * @returns {boolean} true if the value is ignored
          */
         function isIgnoredValue(value) {
-            return ignore.has(value);
+            return ignore.indexOf(value) !== -1;
         }
 
         /**
@@ -209,7 +209,7 @@ module.exports = {
                         });
                     }
                 } else if (
-                    !okTypes.includes(parent.type) ||
+                    okTypes.indexOf(parent.type) === -1 ||
                     (parent.type === "AssignmentExpression" && parent.left.type === "Identifier")
                 ) {
                     context.report({
diff --git a/lib/rules/no-mixed-operators.js b/lib/rules/no-mixed-operators.js
index 0cace7ea910..c75d01147c9 100644
--- a/lib/rules/no-mixed-operators.js
+++ b/lib/rules/no-mixed-operators.js
@@ -64,7 +64,7 @@ function normalizeOptions(options = {}) {
  * @returns {boolean} `true` if such group existed.
  */
 function includesBothInAGroup(groups, left, right) {
-    return groups.some(group => group.includes(left) && group.includes(right));
+    return groups.some(group => group.indexOf(left) !== -1 && group.indexOf(right) !== -1);
 }
 
 /**
diff --git a/lib/rules/no-mixed-requires.js b/lib/rules/no-mixed-requires.js
index 97064243ed3..5e05cf7a72a 100644
--- a/lib/rules/no-mixed-requires.js
+++ b/lib/rules/no-mixed-requires.js
@@ -160,7 +160,7 @@ module.exports = {
                 return REQ_COMPUTED;
             }
 
-            if (BUILTIN_MODULES.includes(arg.value)) {
+            if (BUILTIN_MODULES.indexOf(arg.value) !== -1) {
 
                 // "var fs = require('fs');"
                 return REQ_CORE;
diff --git a/lib/rules/no-multi-spaces.js b/lib/rules/no-multi-spaces.js
index 6fac7e3ca7e..e37aae0a16e 100644
--- a/lib/rules/no-multi-spaces.js
+++ b/lib/rules/no-multi-spaces.js
@@ -56,7 +56,7 @@ module.exports = {
         const options = context.options[0] || {};
         const ignoreEOLComments = options.ignoreEOLComments;
         const exceptions = Object.assign({ Property: true }, options.exceptions);
-        const hasExceptions = Object.keys(exceptions).some(key => exceptions[key]);
+        const hasExceptions = Object.keys(exceptions).filter(key => exceptions[key]).length > 0;
 
         /**
          * Formats value of given comment token for error message by truncating its length.
diff --git a/lib/rules/no-native-reassign.js b/lib/rules/no-native-reassign.js
index 5f396e404c6..1814a5b3c5b 100644
--- a/lib/rules/no-native-reassign.js
+++ b/lib/rules/no-native-reassign.js
@@ -81,7 +81,7 @@ module.exports = {
          * @returns {void}
          */
         function checkVariable(variable) {
-            if (variable.writeable === false && !exceptions.includes(variable.name)) {
+            if (variable.writeable === false && exceptions.indexOf(variable.name) === -1) {
                 variable.references.forEach(checkReference);
             }
         }
diff --git a/lib/rules/no-new-wrappers.js b/lib/rules/no-new-wrappers.js
index 1fe06c1943e..4667d581f65 100644
--- a/lib/rules/no-new-wrappers.js
+++ b/lib/rules/no-new-wrappers.js
@@ -34,7 +34,7 @@ module.exports = {
             NewExpression(node) {
                 const wrapperObjects = ["String", "Number", "Boolean"];
 
-                if (wrapperObjects.includes(node.callee.name)) {
+                if (wrapperObjects.indexOf(node.callee.name) > -1) {
                     context.report({
                         node,
                         messageId: "noConstructor",
diff --git a/lib/rules/no-prototype-builtins.js b/lib/rules/no-prototype-builtins.js
index dc12669c87d..ae736981e71 100644
--- a/lib/rules/no-prototype-builtins.js
+++ b/lib/rules/no-prototype-builtins.js
@@ -33,11 +33,11 @@ module.exports = {
     },
 
     create(context) {
-        const DISALLOWED_PROPS = new Set([
+        const DISALLOWED_PROPS = [
             "hasOwnProperty",
             "isPrototypeOf",
             "propertyIsEnumerable"
-        ]);
+        ];
 
         /**
          * Reports if a disallowed property is used in a CallExpression
@@ -54,7 +54,7 @@ module.exports = {
 
             const propName = astUtils.getStaticPropertyName(callee);
 
-            if (propName !== null && DISALLOWED_PROPS.has(propName)) {
+            if (propName !== null && DISALLOWED_PROPS.indexOf(propName) > -1) {
                 context.report({
                     messageId: "prototypeBuildIn",
                     loc: callee.property.loc,
diff --git a/lib/rules/no-shadow.js b/lib/rules/no-shadow.js
index b4ef334efdd..43d7d738e29 100644
--- a/lib/rules/no-shadow.js
+++ b/lib/rules/no-shadow.js
@@ -15,8 +15,8 @@ const astUtils = require("./utils/ast-utils");
 // Helpers
 //------------------------------------------------------------------------------
 
-const FUNC_EXPR_NODE_TYPES = new Set(["ArrowFunctionExpression", "FunctionExpression"]);
-const CALL_EXPR_NODE_TYPE = new Set(["CallExpression"]);
+const FUNC_EXPR_NODE_TYPES = ["ArrowFunctionExpression", "FunctionExpression"];
+const CALL_EXPR_NODE_TYPE = ["CallExpression"];
 const FOR_IN_OF_TYPE = /^For(?:In|Of)Statement$/u;
 const SENTINEL_TYPE = /^(?:(?:Function|Class)(?:Declaration|Expression)|ArrowFunctionExpression|CatchClause|ImportDeclaration|ExportNamedDeclaration)$/u;
 
@@ -123,7 +123,7 @@ module.exports = {
             const { variableScope } = variable.scope;
 
 
-            if (!(FUNC_EXPR_NODE_TYPES.has(variableScope.block.type) && getOuterScope(variableScope) === shadowedVariable.scope)) {
+            if (!(FUNC_EXPR_NODE_TYPES.includes(variableScope.block.type) && getOuterScope(variableScope) === shadowedVariable.scope)) {
                 return false;
             }
 
@@ -132,7 +132,7 @@ module.exports = {
 
             const callExpression = findSelfOrAncestor(
                 parent,
-                node => CALL_EXPR_NODE_TYPE.has(node.type)
+                node => CALL_EXPR_NODE_TYPE.includes(node.type)
             );
 
             if (!callExpression) {
@@ -173,7 +173,7 @@ module.exports = {
          * @returns {boolean} Whether or not the variable name is allowed.
          */
         function isAllowed(variable) {
-            return options.allow.includes(variable.name);
+            return options.allow.indexOf(variable.name) !== -1;
         }
 
         /**
diff --git a/lib/rules/no-sparse-arrays.js b/lib/rules/no-sparse-arrays.js
index ff5c2cf9978..c95bc203c4a 100644
--- a/lib/rules/no-sparse-arrays.js
+++ b/lib/rules/no-sparse-arrays.js
@@ -37,7 +37,7 @@ module.exports = {
 
             ArrayExpression(node) {
 
-                const emptySpot = node.elements.includes(null);
+                const emptySpot = node.elements.indexOf(null) > -1;
 
                 if (emptySpot) {
                     context.report({ node, messageId: "unexpectedSparseArray" });
diff --git a/lib/rules/no-underscore-dangle.js b/lib/rules/no-underscore-dangle.js
index a3a9f650032..55fb9b8cfbc 100644
--- a/lib/rules/no-underscore-dangle.js
+++ b/lib/rules/no-underscore-dangle.js
@@ -86,7 +86,7 @@ module.exports = {
          * @private
          */
         function isAllowed(identifier) {
-            return ALLOWED_VARIABLES.includes(identifier);
+            return ALLOWED_VARIABLES.some(ident => ident === identifier);
         }
 
         /**
diff --git a/lib/rules/no-unused-expressions.js b/lib/rules/no-unused-expressions.js
index e90099d6201..68a7ebd5e00 100644
--- a/lib/rules/no-unused-expressions.js
+++ b/lib/rules/no-unused-expressions.js
@@ -124,7 +124,7 @@ module.exports = {
              */
             return (parent.type === "Program" || parent.type === "BlockStatement" &&
                     (/Function/u.test(grandparent.type))) &&
-                    directives(parent).includes(node);
+                    directives(parent).indexOf(node) >= 0;
         }
 
         /**
diff --git a/lib/rules/no-unused-vars.js b/lib/rules/no-unused-vars.js
index 11b1f3722de..54e33705f7f 100644
--- a/lib/rules/no-unused-vars.js
+++ b/lib/rules/no-unused-vars.js
@@ -259,7 +259,7 @@ module.exports = {
             let scope = ref.from;
 
             while (scope) {
-                if (nodes.includes(scope.block)) {
+                if (nodes.indexOf(scope.block) >= 0) {
                     return true;
                 }
 
diff --git a/lib/rules/operator-assignment.js b/lib/rules/operator-assignment.js
index 8b9fb5bd73b..50cdff9f9a0 100644
--- a/lib/rules/operator-assignment.js
+++ b/lib/rules/operator-assignment.js
@@ -22,7 +22,7 @@ const astUtils = require("./utils/ast-utils");
  *     shorthand form.
  */
 function isCommutativeOperatorWithShorthand(operator) {
-    return ["*", "&", "^", "|"].includes(operator);
+    return ["*", "&", "^", "|"].indexOf(operator) >= 0;
 }
 
 /**
@@ -33,7 +33,7 @@ function isCommutativeOperatorWithShorthand(operator) {
  *     a shorthand form.
  */
 function isNonCommutativeOperatorWithShorthand(operator) {
-    return ["+", "-", "/", "%", "<<", ">>", ">>>", "**"].includes(operator);
+    return ["+", "-", "/", "%", "<<", ">>", ">>>", "**"].indexOf(operator) >= 0;
 }
 
 //------------------------------------------------------------------------------
diff --git a/lib/rules/prefer-const.js b/lib/rules/prefer-const.js
index cf07d6ce714..190860019d8 100644
--- a/lib/rules/prefer-const.js
+++ b/lib/rules/prefer-const.js
@@ -60,7 +60,7 @@ function canBecomeVariableDeclaration(identifier) {
  */
 function isOuterVariableInDestructing(name, initScope) {
 
-    if (initScope.through.some(ref => ref.resolved && ref.resolved.name === name)) {
+    if (initScope.through.find(ref => ref.resolved && ref.resolved.name === name)) {
         return true;
     }
 
diff --git a/lib/rules/prefer-reflect.js b/lib/rules/prefer-reflect.js
index 377268900b4..1367a1848df 100644
--- a/lib/rules/prefer-reflect.js
+++ b/lib/rules/prefer-reflect.js
@@ -106,7 +106,7 @@ module.exports = {
                 const methodName = (node.callee.property || {}).name;
                 const isReflectCall = (node.callee.object || {}).name === "Reflect";
                 const hasReflectSubstitute = Object.prototype.hasOwnProperty.call(reflectSubstitutes, methodName);
-                const userConfiguredException = exceptions.includes(methodName);
+                const userConfiguredException = exceptions.indexOf(methodName) !== -1;
 
                 if (hasReflectSubstitute && !isReflectCall && !userConfiguredException) {
                     report(node, existingNames[methodName], reflectSubstitutes[methodName]);
@@ -115,7 +115,7 @@ module.exports = {
             UnaryExpression(node) {
                 const isDeleteOperator = node.operator === "delete";
                 const targetsIdentifier = node.argument.type === "Identifier";
-                const userConfiguredException = exceptions.includes("delete");
+                const userConfiguredException = exceptions.indexOf("delete") !== -1;
 
                 if (isDeleteOperator && !targetsIdentifier && !userConfiguredException) {
                     report(node, "the delete keyword", "Reflect.deleteProperty");
diff --git a/lib/rules/prefer-regex-literals.js b/lib/rules/prefer-regex-literals.js
index aa7258997c5..158f84b940d 100644
--- a/lib/rules/prefer-regex-literals.js
+++ b/lib/rules/prefer-regex-literals.js
@@ -47,7 +47,7 @@ function isStaticTemplateLiteral(node) {
     return node.type === "TemplateLiteral" && node.expressions.length === 0;
 }
 
-const validPrecedingTokens = new Set([
+const validPrecedingTokens = [
     "(",
     ";",
     "[",
@@ -110,7 +110,7 @@ const validPrecedingTokens = new Set([
     "debugger",
     "case",
     "throw"
-]);
+];
 
 
 //------------------------------------------------------------------------------
@@ -334,7 +334,7 @@ module.exports = {
 
                         const tokenBefore = sourceCode.getTokenBefore(node);
 
-                        if (tokenBefore && !validPrecedingTokens.has(tokenBefore.value)) {
+                        if (tokenBefore && !validPrecedingTokens.includes(tokenBefore.value)) {
                             noFix = true;
                         }
 
diff --git a/lib/rules/quote-props.js b/lib/rules/quote-props.js
index 0b66d761c47..e57ad6eea29 100644
--- a/lib/rules/quote-props.js
+++ b/lib/rules/quote-props.js
@@ -95,7 +95,7 @@ module.exports = {
          * @returns {boolean} `true` if it is an ES3 token.
          */
         function isKeyword(tokenStr) {
-            return keywords.includes(tokenStr);
+            return keywords.indexOf(tokenStr) >= 0;
         }
 
         /**
@@ -108,7 +108,7 @@ module.exports = {
          */
         function areQuotesRedundant(rawKey, tokens, skipNumberLiterals) {
             return tokens.length === 1 && tokens[0].start === 0 && tokens[0].end === rawKey.length &&
-                (["Identifier", "Keyword", "Null", "Boolean"].includes(tokens[0].type) ||
+                (["Identifier", "Keyword", "Null", "Boolean"].indexOf(tokens[0].type) >= 0 ||
                 (tokens[0].type === "Numeric" && !skipNumberLiterals && String(+tokens[0].value) === tokens[0].value));
         }
 
diff --git a/lib/rules/quotes.js b/lib/rules/quotes.js
index f1d30a044bc..41bff3c4ecf 100644
--- a/lib/rules/quotes.js
+++ b/lib/rules/quotes.js
@@ -283,7 +283,7 @@ module.exports = {
                         astUtils.isSurroundedBy(rawVal, settings.quote);
 
                     if (!isValid && avoidEscape) {
-                        isValid = astUtils.isSurroundedBy(rawVal, settings.alternateQuote) && rawVal.includes(settings.quote);
+                        isValid = astUtils.isSurroundedBy(rawVal, settings.alternateQuote) && rawVal.indexOf(settings.quote) >= 0;
                     }
 
                     if (!isValid) {
diff --git a/lib/rules/spaced-comment.js b/lib/rules/spaced-comment.js
index d858fc47cf8..674c13cf380 100644
--- a/lib/rules/spaced-comment.js
+++ b/lib/rules/spaced-comment.js
@@ -39,7 +39,7 @@ function escapeAndRepeat(s) {
 function parseMarkersOption(markers) {
 
     // `*` is a marker for JSDoc comments.
-    if (!markers.includes("*")) {
+    if (markers.indexOf("*") === -1) {
         return markers.concat("*");
     }
 
diff --git a/lib/rules/valid-jsdoc.js b/lib/rules/valid-jsdoc.js
index 8662bf0eae7..c118e033dd8 100644
--- a/lib/rules/valid-jsdoc.js
+++ b/lib/rules/valid-jsdoc.js
@@ -405,7 +405,7 @@ module.exports = {
                             loc: getAbsoluteRange(jsdocNode, param),
                             data: { name: param.name }
                         });
-                    } else if (!param.name.includes(".")) {
+                    } else if (param.name.indexOf(".") === -1) {
                         paramTagsByName[param.name] = param;
                     }
                 });
diff --git a/lib/rules/valid-typeof.js b/lib/rules/valid-typeof.js
index 2286d8926cc..cb85cd9cb90 100644
--- a/lib/rules/valid-typeof.js
+++ b/lib/rules/valid-typeof.js
@@ -42,8 +42,8 @@ module.exports = {
 
     create(context) {
 
-        const VALID_TYPES = new Set(["symbol", "undefined", "object", "boolean", "number", "string", "function", "bigint"]),
-            OPERATORS = new Set(["==", "===", "!=", "!=="]);
+        const VALID_TYPES = ["symbol", "undefined", "object", "boolean", "number", "string", "function", "bigint"],
+            OPERATORS = ["==", "===", "!=", "!=="];
 
         const requireStringLiterals = context.options[0] && context.options[0].requireStringLiterals;
 
@@ -85,13 +85,13 @@ module.exports = {
                 if (isTypeofExpression(node)) {
                     const parent = context.getAncestors().pop();
 
-                    if (parent.type === "BinaryExpression" && OPERATORS.has(parent.operator)) {
+                    if (parent.type === "BinaryExpression" && OPERATORS.indexOf(parent.operator) !== -1) {
                         const sibling = parent.left === node ? parent.right : parent.left;
 
                         if (sibling.type === "Literal" || sibling.type === "TemplateLiteral" && !sibling.expressions.length) {
                             const value = sibling.type === "Literal" ? sibling.value : sibling.quasis[0].value.cooked;
 
-                            if (!VALID_TYPES.has(value)) {
+                            if (VALID_TYPES.indexOf(value) === -1) {
                                 context.report({ node: sibling, messageId: "invalidValue" });
                             }
                         } else if (sibling.type === "Identifier" && sibling.name === "undefined" && isReferenceToGlobalVariable(sibling)) {
diff --git a/lib/rules/yoda.js b/lib/rules/yoda.js
index 5b64287a151..f491f756d54 100644
--- a/lib/rules/yoda.js
+++ b/lib/rules/yoda.js
@@ -39,7 +39,7 @@ function isEqualityOperator(operator) {
  * @returns {boolean} Whether the operator is used in range tests.
  */
 function isRangeTestOperator(operator) {
-    return ["<", "<="].includes(operator);
+    return ["<", "<="].indexOf(operator) >= 0;
 }
 
 /**
diff --git a/package.json b/package.json
index 380ebf6597d..dbbc034edf6 100644
--- a/package.json
+++ b/package.json
@@ -100,7 +100,6 @@
     "eslint-plugin-internal-rules": "file:tools/internal-rules",
     "eslint-plugin-jsdoc": "^37.0.0",
     "eslint-plugin-node": "^11.1.0",
-    "eslint-plugin-unicorn": "^42.0.0",
     "eslint-release": "^3.2.0",
     "eslump": "^3.0.0",
     "esprima": "^4.0.1",
diff --git a/packages/eslint-config-eslint/default.yml b/packages/eslint-config-eslint/default.yml
index 3985ae2988c..1fcbebe75f6 100644
--- a/packages/eslint-config-eslint/default.yml
+++ b/packages/eslint-config-eslint/default.yml
@@ -4,8 +4,6 @@ extends:
     - "plugin:node/recommended"
     - "plugin:jsdoc/recommended"
     - "plugin:eslint-comments/recommended"
-plugins:
-    - unicorn
 settings:
     jsdoc:
         tagNamePreference:
@@ -281,19 +279,6 @@ rules:
     template-curly-spacing: ["error", "never"]
     template-tag-spacing: "error"
     unicode-bom: "error"
-
-    # Selectively-enable unicorn rules since many of its recommended rules are too aggressive/stylistic for us.
-    unicorn/prefer-array-find: "error"
-    unicorn/prefer-array-flat-map: "error"
-    unicorn/prefer-array-flat: "error"
-    unicorn/prefer-array-index-of: "error"
-    unicorn/prefer-array-some: "error"
-    unicorn/prefer-includes: "error"
-    unicorn/prefer-set-has: "error"
-    unicorn/prefer-string-slice: "error"
-    unicorn/prefer-string-starts-ends-with: "error"
-    unicorn/prefer-string-trim-start-end: "error"
-
     wrap-iife: "error"
     yield-star-spacing: "error"
     yoda: ["error", "never", { exceptRange: true }]
diff --git a/packages/eslint-config-eslint/package.json b/packages/eslint-config-eslint/package.json
index e64f65a7a9b..faa9f4a7188 100644
--- a/packages/eslint-config-eslint/package.json
+++ b/packages/eslint-config-eslint/package.json
@@ -22,8 +22,7 @@
   "peerDependencies": {
     "eslint-plugin-eslint-comments": ">=3.2.0",
     "eslint-plugin-jsdoc": ">=36.0.6",
-    "eslint-plugin-node": ">=11.1.0",
-    "eslint-plugin-unicorn": ">=42.0.0"
+    "eslint-plugin-node": ">=11.1.0"
   },
   "keywords": [
     "eslintconfig",
diff --git a/tests/lib/cli.js b/tests/lib/cli.js
index d1725c05b72..59ff0eaf7a4 100644
--- a/tests/lib/cli.js
+++ b/tests/lib/cli.js
@@ -552,9 +552,9 @@ describe("cli", () => {
             const call = log.info.getCall(0);
 
             assert.isTrue(log.info.calledOnce);
-            assert.isTrue(call.args[0].includes("String!"));
-            assert.isTrue(call.args[0].includes("Literal!"));
-            assert.isTrue(call.args[0].includes("2 problems"));
+            assert.isTrue(call.args[0].indexOf("String!") > -1);
+            assert.isTrue(call.args[0].indexOf("Literal!") > -1);
+            assert.isTrue(call.args[0].indexOf("2 problems") > -1);
             assert.isTrue(log.info.neverCalledWith(""));
             assert.strictEqual(exit, 1);
         });
diff --git a/tests/lib/linter/node-event-generator.js b/tests/lib/linter/node-event-generator.js
index 4004f7a2835..44f8e512d67 100644
--- a/tests/lib/linter/node-event-generator.js
+++ b/tests/lib/linter/node-event-generator.js
@@ -126,7 +126,7 @@ describe("NodeEventGenerator", () => {
             it(possibleQueries.join("; "), () => {
                 const ast = espree.parse(sourceText, ESPREE_CONFIG);
                 const emissions = getEmissions(ast, possibleQueries)
-                    .filter(emission => possibleQueries.includes(emission[0]));
+                    .filter(emission => possibleQueries.indexOf(emission[0]) !== -1);
 
                 assert.deepStrictEqual(emissions, expectedEmissions(ast));
             });
@@ -388,7 +388,7 @@ describe("NodeEventGenerator", () => {
         function assertEmissions(ast, visitorKeys, possibleQueries, expectedEmissions) {
             it(possibleQueries.join("; "), () => {
                 const emissions = getEmissions(ast, visitorKeys, possibleQueries)
-                    .filter(emission => possibleQueries.includes(emission[0]));
+                    .filter(emission => possibleQueries.indexOf(emission[0]) !== -1);
 
                 assert.deepStrictEqual(emissions, expectedEmissions(ast));
             });
diff --git a/tests/lib/rules/no-invalid-this.js b/tests/lib/rules/no-invalid-this.js
index dd72be49991..b106a706c0f 100644
--- a/tests/lib/rules/no-invalid-this.js
+++ b/tests/lib/rules/no-invalid-this.js
@@ -86,7 +86,8 @@ function extractPatterns(patterns, type) {
         return thisPattern;
     }));
 
-    return patternsList.flat();
+    // Flatten.
+    return [].concat(...patternsList);
 }
 
 //------------------------------------------------------------------------------
diff --git a/tools/code-sample-minimizer.js b/tools/code-sample-minimizer.js
index e1eb4995cc0..c68f4582050 100644
--- a/tools/code-sample-minimizer.js
+++ b/tools/code-sample-minimizer.js
@@ -133,8 +133,10 @@ function reduceBadExampleSize({
      * @returns {ASTNode} A descendent of `node` which is also bad
      */
     function extractRelevantChild(node) {
-        const childNodes = visitorKeys[node.type]
-            .flatMap(key => (Array.isArray(node[key]) ? node[key] : [node[key]]));
+        const childNodes = [].concat(
+            ...visitorKeys[node.type]
+                .map(key => (Array.isArray(node[key]) ? node[key] : [node[key]]))
+        );
 
         for (const childNode of childNodes) {
             if (!childNode) {