diff --git a/src/jsutils/inspect.js b/src/jsutils/inspect.js
index 03cc2d7b5f..b4cebc4a8a 100644
--- a/src/jsutils/inspect.js
+++ b/src/jsutils/inspect.js
@@ -31,7 +31,7 @@ function formatObjectValue(
     return 'null';
   }
 
-  if (previouslySeenValues.indexOf(value) !== -1) {
+  if (previouslySeenValues.includes(value)) {
     return '[Circular]';
   }
 
diff --git a/src/language/blockString.js b/src/language/blockString.js
index 08f106005e..013cb60011 100644
--- a/src/language/blockString.js
+++ b/src/language/blockString.js
@@ -95,7 +95,7 @@ export function printBlockString(
   value: string,
   preferMultipleLines: boolean = false,
 ): string {
-  const isSingleLine = value.indexOf('\n') === -1;
+  const isSingleLine = !value.includes('\n');
   const hasLeadingSpace = value[0] === ' ' || value[0] === '\t';
   const hasTrailingQuote = value[value.length - 1] === '"';
   const hasTrailingSlash = value[value.length - 1] === '\\';
diff --git a/src/language/printer.js b/src/language/printer.js
index 334742d967..4877072969 100644
--- a/src/language/printer.js
+++ b/src/language/printer.js
@@ -332,10 +332,7 @@ function indent(str: string): string {
   return wrap('  ', str.replace(/\n/g, '\n  '));
 }
 
-function isMultiline(str: string): boolean {
-  return str.indexOf('\n') !== -1;
-}
-
 function hasMultilineItems(maybeArray: ?Array<string>): boolean {
-  return maybeArray != null && maybeArray.some(isMultiline);
+  // istanbul ignore next (See: 'https://github.com/graphql/graphql-js/issues/2203')
+  return maybeArray?.some((str) => str.includes('\n')) ?? false;
 }
diff --git a/src/type/validate.js b/src/type/validate.js
index fdaa5f8e45..4d39256ac4 100644
--- a/src/type/validate.js
+++ b/src/type/validate.js
@@ -441,7 +441,7 @@ function validateTypeImplementsAncestors(
 ): void {
   const ifaceInterfaces = type.getInterfaces();
   for (const transitive of iface.getInterfaces()) {
-    if (ifaceInterfaces.indexOf(transitive) === -1) {
+    if (!ifaceInterfaces.includes(transitive)) {
       context.reportError(
         transitive === type
           ? `Type ${type.name} cannot implement ${iface.name} because it would create a circular reference.`
diff --git a/src/utilities/__tests__/stripIgnoredCharacters-test.js b/src/utilities/__tests__/stripIgnoredCharacters-test.js
index e73fa56362..293258707c 100644
--- a/src/utilities/__tests__/stripIgnoredCharacters-test.js
+++ b/src/utilities/__tests__/stripIgnoredCharacters-test.js
@@ -370,7 +370,7 @@ describe('stripIgnoredCharacters', () => {
     expectStripped('""",|"""').toStayTheSame();
 
     const ignoredTokensWithoutFormatting = ignoredTokens.filter(
-      (token) => ['\n', '\r', '\r\n', '\t', ' '].indexOf(token) === -1,
+      (token) => !['\n', '\r', '\r\n', '\t', ' '].includes(token),
     );
     for (const ignored of ignoredTokensWithoutFormatting) {
       expectStripped('"""|' + ignored + '|"""').toStayTheSame();
diff --git a/src/utilities/findBreakingChanges.js b/src/utilities/findBreakingChanges.js
index 8fb29c87c1..2a13d4655b 100644
--- a/src/utilities/findBreakingChanges.js
+++ b/src/utilities/findBreakingChanges.js
@@ -156,7 +156,7 @@ function findDirectiveChanges(
     }
 
     for (const location of oldDirective.locations) {
-      if (newDirective.locations.indexOf(location) === -1) {
+      if (!newDirective.locations.includes(location)) {
         schemaChanges.push({
           type: BreakingChangeType.DIRECTIVE_LOCATION_REMOVED,
           description: `${location} was removed from ${oldDirective.name}.`,
diff --git a/src/validation/rules/KnownArgumentNamesRule.js b/src/validation/rules/KnownArgumentNamesRule.js
index dbd8fb82fe..d769fe4966 100644
--- a/src/validation/rules/KnownArgumentNamesRule.js
+++ b/src/validation/rules/KnownArgumentNamesRule.js
@@ -78,7 +78,7 @@ export function KnownArgumentNamesOnDirectivesRule(
       if (directiveNode.arguments && knownArgs) {
         for (const argNode of directiveNode.arguments) {
           const argName = argNode.name.value;
-          if (knownArgs.indexOf(argName) === -1) {
+          if (!knownArgs.includes(argName)) {
             const suggestions = suggestionList(argName, knownArgs);
             context.reportError(
               new GraphQLError(
diff --git a/src/validation/rules/KnownDirectivesRule.js b/src/validation/rules/KnownDirectivesRule.js
index 05a0d3b261..8c1e9191d1 100644
--- a/src/validation/rules/KnownDirectivesRule.js
+++ b/src/validation/rules/KnownDirectivesRule.js
@@ -55,7 +55,7 @@ export function KnownDirectivesRule(
       }
 
       const candidateLocation = getDirectiveLocationForASTPath(ancestors);
-      if (candidateLocation && locations.indexOf(candidateLocation) === -1) {
+      if (candidateLocation && !locations.includes(candidateLocation)) {
         context.reportError(
           new GraphQLError(
             `Directive "@${name}" may not be used on ${candidateLocation}.`,
diff --git a/src/validation/rules/KnownTypeNamesRule.js b/src/validation/rules/KnownTypeNamesRule.js
index 3c49b3840a..bf443861a2 100644
--- a/src/validation/rules/KnownTypeNamesRule.js
+++ b/src/validation/rules/KnownTypeNamesRule.js
@@ -48,7 +48,7 @@ export function KnownTypeNamesRule(
       if (!existingTypesMap[typeName] && !definedTypes[typeName]) {
         const definitionNode = ancestors[2] ?? parent;
         const isSDL = definitionNode != null && isSDLNode(definitionNode);
-        if (isSDL && isStandardTypeName(typeName)) {
+        if (isSDL && standardTypeNames.includes(typeName)) {
           return;
         }
 
@@ -71,10 +71,6 @@ const standardTypeNames = [...specifiedScalarTypes, ...introspectionTypes].map(
   (type) => type.name,
 );
 
-function isStandardTypeName(typeName: string): boolean {
-  return standardTypeNames.indexOf(typeName) !== -1;
-}
-
 function isSDLNode(value: ASTNode | $ReadOnlyArray<ASTNode>): boolean {
   return (
     !Array.isArray(value) &&