diff --git a/src/__fixtures__/index.js b/src/__fixtures__/index.js
index ff912f40a5..460bee30f0 100644
--- a/src/__fixtures__/index.js
+++ b/src/__fixtures__/index.js
@@ -3,14 +3,16 @@
 import { join } from 'path';
 import { readFileSync } from 'fs';
 
-function readLocalFile(filename) {
+function readLocalFile(filename: string): string {
   return readFileSync(join(__dirname, filename), 'utf8');
 }
 
-export const bigSchemaSDL = readLocalFile('github-schema.graphql');
-export const bigSchemaIntrospectionResult = JSON.parse(
+export const bigSchemaSDL: string = readLocalFile('github-schema.graphql');
+export const bigSchemaIntrospectionResult: any = JSON.parse(
   readLocalFile('github-schema.json'),
 );
 
-export const kitchenSinkSDL = readLocalFile('schema-kitchen-sink.graphql');
-export const kitchenSinkQuery = readLocalFile('kitchen-sink.graphql');
+export const kitchenSinkSDL: string = readLocalFile(
+  'schema-kitchen-sink.graphql',
+);
+export const kitchenSinkQuery: string = readLocalFile('kitchen-sink.graphql');
diff --git a/src/__tests__/starWarsSchema.js b/src/__tests__/starWarsSchema.js
index 37fe0ada36..adac1a5ce2 100644
--- a/src/__tests__/starWarsSchema.js
+++ b/src/__tests__/starWarsSchema.js
@@ -293,7 +293,7 @@ const queryType = new GraphQLObjectType({
  * Finally, we construct our schema (whose starting query type is the query
  * type we defined above) and export it.
  */
-export const StarWarsSchema = new GraphQLSchema({
+export const StarWarsSchema: GraphQLSchema = new GraphQLSchema({
   query: queryType,
   types: [humanType, droidType],
 });
diff --git a/src/jsutils/Path.js b/src/jsutils/Path.js
index 17f78e6c77..f477354e07 100644
--- a/src/jsutils/Path.js
+++ b/src/jsutils/Path.js
@@ -8,7 +8,10 @@ export type Path = {|
 /**
  * Given a Path and a key, return a new Path containing the new key.
  */
-export function addPath(prev: $ReadOnly<Path> | void, key: string | number) {
+export function addPath(
+  prev: $ReadOnly<Path> | void,
+  key: string | number,
+): Path {
   return { prev, key };
 }
 
diff --git a/src/language/lexer.js b/src/language/lexer.js
index a9601bdfb5..58e3a0d419 100644
--- a/src/language/lexer.js
+++ b/src/language/lexer.js
@@ -76,7 +76,7 @@ export class Lexer {
 /**
  * @internal
  */
-export function isPunctuatorTokenKind(kind: TokenKindEnum) {
+export function isPunctuatorTokenKind(kind: TokenKindEnum): boolean %checks {
   return (
     kind === TokenKind.BANG ||
     kind === TokenKind.DOLLAR ||
diff --git a/src/language/visitor.js b/src/language/visitor.js
index ef3d533fee..68985d9c12 100644
--- a/src/language/visitor.js
+++ b/src/language/visitor.js
@@ -48,7 +48,7 @@ export type VisitorKeyMap<KindToNode> = $ObjMap<
   <T>(T) => $ReadOnlyArray<$Keys<T>>,
 >;
 
-export const QueryDocumentKeys = {
+export const QueryDocumentKeys: VisitorKeyMap<ASTKindToNode> = {
   Name: [],
 
   Document: ['definitions'],
@@ -135,7 +135,7 @@ export const QueryDocumentKeys = {
   InputObjectTypeExtension: ['name', 'directives', 'fields'],
 };
 
-export const BREAK = Object.freeze({});
+export const BREAK: { ... } = Object.freeze({});
 
 /**
  * visit() will walk through an AST using a depth first traversal, calling
@@ -363,7 +363,7 @@ export function visitInParallel(
   return {
     enter(node) {
       for (let i = 0; i < visitors.length; i++) {
-        if (!skipping[i]) {
+        if (skipping[i] == null) {
           const fn = getVisitFn(visitors[i], node.kind, /* isLeaving */ false);
           if (fn) {
             const result = fn.apply(visitors[i], arguments);
@@ -380,7 +380,7 @@ export function visitInParallel(
     },
     leave(node) {
       for (let i = 0; i < visitors.length; i++) {
-        if (!skipping[i]) {
+        if (skipping[i] == null) {
           const fn = getVisitFn(visitors[i], node.kind, /* isLeaving */ true);
           if (fn) {
             const result = fn.apply(visitors[i], arguments);