diff --git a/src/jsutils/isAsyncIterable.ts b/src/jsutils/isAsyncIterable.ts
index 5664996ad85..608230e7777 100644
--- a/src/jsutils/isAsyncIterable.ts
+++ b/src/jsutils/isAsyncIterable.ts
@@ -2,10 +2,7 @@
  * Returns true if the provided object implements the AsyncIterator protocol via
  * either implementing a `Symbol.asyncIterator` or `"@@asyncIterator"` method.
  */
-declare function isAsyncIterable(value: unknown): boolean; // FIXME: TS_CONVERTION %check 
-
-// eslint-disable-next-line no-redeclare
-export default function isAsyncIterable(maybeAsyncIterable) {
+export default function isAsyncIterable(maybeAsyncIterable:unknown): maybeAsyncIterable is AsyncIterable<unknown> {
   if (maybeAsyncIterable == null || typeof maybeAsyncIterable !== 'object') {
     return false;
   }
diff --git a/src/jsutils/isCollection.ts b/src/jsutils/isCollection.ts
index d6df99eb661..1b8695b0b85 100644
--- a/src/jsutils/isCollection.ts
+++ b/src/jsutils/isCollection.ts
@@ -19,10 +19,7 @@
  *   An Object value which might implement the Iterable or Array-like protocols.
  * @return {boolean} true if Iterable or Array-like Object.
  */
-declare function isCollection(value: unknown): boolean;  // FIXME: TS_CONVERTION %check 
-
-// eslint-disable-next-line no-redeclare
-export default function isCollection(obj) {
+export default function isCollection(obj: unknown): obj is object| Symbol {
   if (obj == null || typeof obj !== 'object') {
     return false;
   }
diff --git a/src/jsutils/isObjectLike.ts b/src/jsutils/isObjectLike.ts
index 35486c50c71..9331c48f3e1 100644
--- a/src/jsutils/isObjectLike.ts
+++ b/src/jsutils/isObjectLike.ts
@@ -2,6 +2,6 @@
  * Return true if `value` is object-like. A value is object-like if it's not
  * `null` and has a `typeof` result of "object".
  */
-export default function isObjectLike(value: unknown): boolean {  // FIXME: TS_CONVERTION %check
+export default function isObjectLike(value: unknown): value is object {
   return typeof value == 'object' && value !== null;
 }
diff --git a/src/jsutils/isPromise.ts b/src/jsutils/isPromise.ts
index e44d20f19de..36a0eaef417 100644
--- a/src/jsutils/isPromise.ts
+++ b/src/jsutils/isPromise.ts
@@ -2,9 +2,6 @@
  * Returns true if the value acts like a Promise, i.e. has a "then" function,
  * otherwise returns false.
  */
-declare function isPromise(value: unknown): boolean;  // FIXME: TS_CONVERTION %check
-
-// eslint-disable-next-line no-redeclare
-export default function isPromise(value) {
+export default function isPromise(value: unknown): value is Promise<unknown> {
   return typeof value?.then === 'function';
 }
diff --git a/src/language/ast.ts b/src/language/ast.ts
index 8b8cba9a298..b4edb66c3b4 100644
--- a/src/language/ast.ts
+++ b/src/language/ast.ts
@@ -136,7 +136,7 @@ export class Token {
 /**
  * @internal
  */
-export function isNode(maybeNode: unknown): boolean {  // FIXME: TS_CONVERTION %check
+export function isNode(maybeNode: unknown): maybeNode is ASTNode {
   return maybeNode != null && typeof maybeNode.kind === 'string';
 }
 
diff --git a/src/language/lexer.ts b/src/language/lexer.ts
index b39818d3f25..1204e6e0485 100644
--- a/src/language/lexer.ts
+++ b/src/language/lexer.ts
@@ -75,7 +75,7 @@ export class Lexer {
 /**
  * @internal
  */
-export function isPunctuatorTokenKind(kind: TokenKindEnum): boolean {
+export function isPunctuatorTokenKind(kind: TokenKindEnum): kind is typeof TokenKind {
   return (
     kind === TokenKind.BANG ||
     kind === TokenKind.DOLLAR ||
diff --git a/src/language/predicates.ts b/src/language/predicates.ts
index 5ca81a36517..a85c4745814 100644
--- a/src/language/predicates.ts
+++ b/src/language/predicates.ts
@@ -1,7 +1,18 @@
-import type { ASTNode } from './ast';
+import {
+  ASTNode,
+  DefinitionNode,
+  ExecutableDefinitionNode,
+  SelectionNode,
+  ValueNode,
+  TypeNode,
+  TypeSystemDefinitionNode,
+  TypeDefinitionNode,
+  TypeSystemExtensionNode,
+  TypeExtensionNode,
+} from './ast';
 import { Kind } from './kinds';
 
-export function isDefinitionNode(node: ASTNode): boolean {
+export function isDefinitionNode(node: ASTNode): node is DefinitionNode {
   return (
     isExecutableDefinitionNode(node) ||
     isTypeSystemDefinitionNode(node) ||
@@ -9,14 +20,14 @@ export function isDefinitionNode(node: ASTNode): boolean {
   );
 }
 
-export function isExecutableDefinitionNode(node: ASTNode): boolean {
+export function isExecutableDefinitionNode(node: ASTNode): node is ExecutableDefinitionNode {
   return (
     node.kind === Kind.OPERATION_DEFINITION ||
     node.kind === Kind.FRAGMENT_DEFINITION
   );
 }
 
-export function isSelectionNode(node: ASTNode): boolean {
+export function isSelectionNode(node: ASTNode): node is SelectionNode {
   return (
     node.kind === Kind.FIELD ||
     node.kind === Kind.FRAGMENT_SPREAD ||
@@ -24,7 +35,7 @@ export function isSelectionNode(node: ASTNode): boolean {
   );
 }
 
-export function isValueNode(node: ASTNode): boolean {
+export function isValueNode(node: ASTNode): node is ValueNode {
   return (
     node.kind === Kind.VARIABLE ||
     node.kind === Kind.INT ||
@@ -38,7 +49,7 @@ export function isValueNode(node: ASTNode): boolean {
   );
 }
 
-export function isTypeNode(node: ASTNode): boolean {
+export function isTypeNode(node: ASTNode): node is TypeNode {
   return (
     node.kind === Kind.NAMED_TYPE ||
     node.kind === Kind.LIST_TYPE ||
@@ -46,7 +57,7 @@ export function isTypeNode(node: ASTNode): boolean {
   );
 }
 
-export function isTypeSystemDefinitionNode(node: ASTNode): boolean {
+export function isTypeSystemDefinitionNode(node: ASTNode): node is TypeSystemDefinitionNode {
   return (
     node.kind === Kind.SCHEMA_DEFINITION ||
     isTypeDefinitionNode(node) ||
@@ -54,7 +65,7 @@ export function isTypeSystemDefinitionNode(node: ASTNode): boolean {
   );
 }
 
-export function isTypeDefinitionNode(node: ASTNode): boolean {
+export function isTypeDefinitionNode(node: ASTNode): node is TypeDefinitionNode {
   return (
     node.kind === Kind.SCALAR_TYPE_DEFINITION ||
     node.kind === Kind.OBJECT_TYPE_DEFINITION ||
@@ -65,11 +76,11 @@ export function isTypeDefinitionNode(node: ASTNode): boolean {
   );
 }
 
-export function isTypeSystemExtensionNode(node: ASTNode): boolean {
+export function isTypeSystemExtensionNode(node: ASTNode): node is TypeSystemExtensionNode {
   return node.kind === Kind.SCHEMA_EXTENSION || isTypeExtensionNode(node);
 }
 
-export function isTypeExtensionNode(node: ASTNode): boolean {
+export function isTypeExtensionNode(node: ASTNode): node is TypeExtensionNode {
   return (
     node.kind === Kind.SCALAR_TYPE_EXTENSION ||
     node.kind === Kind.OBJECT_TYPE_EXTENSION ||
diff --git a/src/language/source.ts b/src/language/source.ts
index e737bf7ec8c..ea665f2692d 100644
--- a/src/language/source.ts
+++ b/src/language/source.ts
@@ -53,8 +53,6 @@ export class Source {
  *
  * @internal
  */
-declare function isSource(source: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isSource(source) {
+export function isSource(source: unknown): source is Source {
   return instanceOf(source, Source);
 }
diff --git a/src/type/definition.ts b/src/type/definition.ts
index 5a38dc6396c..d3da0ae21b6 100644
--- a/src/type/definition.ts
+++ b/src/type/definition.ts
@@ -65,7 +65,7 @@ export type GraphQLType =
   | GraphQLList<any>
   | GraphQLNonNull<any>;
 
-export function isType(type: unknown): boolean {
+export function isType(type: unknown): type is GraphQLType {
   return (
     isScalarType(type) ||
     isObjectType(type) ||
@@ -88,10 +88,7 @@ export function assertType(type: unknown): GraphQLType {
 /**
  * There are predicates for each kind of GraphQL type.
  */
-
-declare function isScalarType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isScalarType(type) {
+export function isScalarType(type: unknown): type is GraphQLScalarType {
   return instanceOf(type, GraphQLScalarType);
 }
 
@@ -102,9 +99,7 @@ export function assertScalarType(type: unknown): GraphQLScalarType {
   return type;
 }
 
-declare function isObjectType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isObjectType(type) {
+export function isObjectType(type: unknown): type is GraphQLObjectType {
   return instanceOf(type, GraphQLObjectType);
 }
 
@@ -115,9 +110,7 @@ export function assertObjectType(type: unknown): GraphQLObjectType {
   return type;
 }
 
-declare function isInterfaceType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isInterfaceType(type) {
+export function isInterfaceType(type: unknown): type is GraphQLInterfaceType {
   return instanceOf(type, GraphQLInterfaceType);
 }
 
@@ -130,9 +123,7 @@ export function assertInterfaceType(type: unknown): GraphQLInterfaceType {
   return type;
 }
 
-declare function isUnionType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isUnionType(type) {
+export function isUnionType(type: unknown): type is GraphQLUnionType {
   return instanceOf(type, GraphQLUnionType);
 }
 
@@ -143,9 +134,7 @@ export function assertUnionType(type: unknown): GraphQLUnionType {
   return type;
 }
 
-declare function isEnumType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isEnumType(type) {
+export function isEnumType(type: unknown): type is GraphQLEnumType {
   return instanceOf(type, GraphQLEnumType);
 }
 
@@ -156,9 +145,7 @@ export function assertEnumType(type: unknown): GraphQLEnumType {
   return type;
 }
 
-declare function isInputObjectType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isInputObjectType(type) {
+export function isInputObjectType(type: unknown): type is GraphQLInputObjectType {
   return instanceOf(type, GraphQLInputObjectType);
 }
 
@@ -171,9 +158,7 @@ export function assertInputObjectType(type: unknown): GraphQLInputObjectType {
   return type;
 }
 
-declare function isListType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isListType(type) {
+export function isListType(type: unknown): type is GraphQLList<any>  {
   return instanceOf(type, GraphQLList);
 }
 
@@ -184,9 +169,7 @@ export function assertListType(type: unknown): GraphQLList<any> {
   return type;
 }
 
-declare function isNonNullType(type: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isNonNullType(type) {
+export function isNonNullType(type: unknown): type is GraphQLNonNull<any> {
   return instanceOf(type, GraphQLNonNull);
 }
 
@@ -212,7 +195,7 @@ export type GraphQLInputType =
       | GraphQLList<GraphQLInputType>,
     >;
 
-export function isInputType(type: unknown): boolean  {
+export function isInputType(type: unknown): type is GraphQLInputType  {
   return (
     isScalarType(type) ||
     isEnumType(type) ||
@@ -247,7 +230,7 @@ export type GraphQLOutputType =
       | GraphQLList<GraphQLOutputType>,
     >;
 
-export function isOutputType(type: unknown): boolean  {
+export function isOutputType(type: unknown): type is GraphQLOutputType  {
   return (
     isScalarType(type) ||
     isObjectType(type) ||
@@ -270,7 +253,7 @@ export function assertOutputType(type: unknown): GraphQLOutputType {
  */
 export type GraphQLLeafType = GraphQLScalarType | GraphQLEnumType;
 
-export function isLeafType(type: unknown): boolean  {
+export function isLeafType(type: unknown): type is GraphQLLeafType  {
   return isScalarType(type) || isEnumType(type);
 }
 
@@ -289,7 +272,7 @@ export type GraphQLCompositeType =
   | GraphQLInterfaceType
   | GraphQLUnionType;
 
-export function isCompositeType(type: unknown): boolean {
+export function isCompositeType(type: unknown): type is GraphQLCompositeType {
   return isObjectType(type) || isInterfaceType(type) || isUnionType(type);
 }
 
@@ -307,7 +290,7 @@ export function assertCompositeType(type: unknown): GraphQLCompositeType {
  */
 export type GraphQLAbstractType = GraphQLInterfaceType | GraphQLUnionType;
 
-export function isAbstractType(type: unknown): boolean {
+export function isAbstractType(type: unknown): type is GraphQLAbstractType {
   return isInterfaceType(type) || isUnionType(type);
 }
 
@@ -414,7 +397,7 @@ export class GraphQLNonNull<T extends GraphQLNullableType> {
 
 export type GraphQLWrappingType = GraphQLList<any> | GraphQLNonNull<any>;
 
-export function isWrappingType(type: unknown): boolean {
+export function isWrappingType(type: unknown): type is GraphQLWrappingType {
   return isListType(type) || isNonNullType(type);
 }
 
@@ -437,7 +420,7 @@ export type GraphQLNullableType =
   | GraphQLInputObjectType
   | GraphQLList<any>;
 
-export function isNullableType(type: unknown): boolean {
+export function isNullableType(type: unknown): type is GraphQLNullableType {
   return isType(type) && !isNonNullType(type);
 }
 
@@ -448,12 +431,11 @@ export function assertNullableType(type: unknown): GraphQLNullableType {
   return type;
 }
 
-/* eslint-disable no-redeclare */
-declare function getNullableType(type: void | null): void;
-declare function getNullableType<T extends GraphQLNullableType>(type: T): T;
-declare function getNullableType<T>(type: GraphQLNonNull<T>): T;
+
+export function getNullableType(type: void | null): void;
+export function getNullableType<T extends GraphQLNullableType>(type: T): T;
+export function getNullableType<T extends GraphQLNullableType>(type: GraphQLNonNull<T>): T;
 export function getNullableType(type) {
-  /* eslint-enable no-redeclare */
   if (type) {
     return isNonNullType(type) ? type.ofType : type;
   }
@@ -470,7 +452,7 @@ export type GraphQLNamedType =
   | GraphQLEnumType
   | GraphQLInputObjectType;
 
-export function isNamedType(type: unknown): boolean {
+export function isNamedType(type: unknown): type is GraphQLNamedType {
   return (
     isScalarType(type) ||
     isObjectType(type) ||
@@ -488,11 +470,9 @@ export function assertNamedType(type: unknown): GraphQLNamedType {
   return type;
 }
 
-/* eslint-disable no-redeclare */
-declare function getNamedType(type: void | null): void;
-declare function getNamedType(type: GraphQLType): GraphQLNamedType;
+export function getNamedType(type: void | null): void;
+export function getNamedType(type: GraphQLType): GraphQLNamedType;
 export function getNamedType(type) {
-  /* eslint-enable no-redeclare */
   if (type) {
     let unwrappedType = type;
     while (isWrappingType(unwrappedType)) {
diff --git a/src/type/directives.ts b/src/type/directives.ts
index 9bc3659f000..dbdcb6b52af 100644
--- a/src/type/directives.ts
+++ b/src/type/directives.ts
@@ -22,11 +22,7 @@ import { Maybe } from '../jsutils/Maybe';
 /**
  * Test if the given value is a GraphQL directive.
  */
-declare function isDirective(
-  directive: unknown,
-): boolean;
-// eslint-disable-next-line no-redeclare
-export function isDirective(directive) {
+export function isDirective(directive: unknown): directive is GraphQLDirective {
   return instanceOf(directive, GraphQLDirective);
 }
 
diff --git a/src/type/introspection.ts b/src/type/introspection.ts
index 0d9b0e2d815..f7aedf17045 100644
--- a/src/type/introspection.ts
+++ b/src/type/introspection.ts
@@ -541,6 +541,6 @@ export const introspectionTypes = Object.freeze([
   __TypeKind,
 ]);
 
-export function isIntrospectionType(type: GraphQLNamedType): boolean { // FIXME: TS_CONVERTION %check
+export function isIntrospectionType(type: GraphQLNamedType): boolean {
   return introspectionTypes.some(({ name }) => type.name === name);
 }
diff --git a/src/type/schema.ts b/src/type/schema.ts
index 299db414612..7e64aa86fd6 100644
--- a/src/type/schema.ts
+++ b/src/type/schema.ts
@@ -43,9 +43,7 @@ import { Maybe } from '../jsutils/Maybe';
 /**
  * Test if the given value is a GraphQL schema.
  */
-declare function isSchema(schema: unknown): boolean;
-// eslint-disable-next-line no-redeclare
-export function isSchema(schema) {
+export function isSchema(schema: unknown): schema is GraphQLSchema {
   return instanceOf(schema, GraphQLSchema);
 }