diff --git a/.eslintrc.yml b/.eslintrc.yml
index 496ee220c2..d8719a90e9 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -84,7 +84,7 @@ rules:
   no-irregular-whitespace: error
   no-misleading-character-class: error
   no-obj-calls: error
-  no-prototype-builtins: off # TODO
+  no-prototype-builtins: error
   no-regex-spaces: error
   no-sparse-arrays: error
   no-template-curly-in-string: error
diff --git a/src/execution/__tests__/variables-test.js b/src/execution/__tests__/variables-test.js
index 4aa8cac98a..f833194328 100644
--- a/src/execution/__tests__/variables-test.js
+++ b/src/execution/__tests__/variables-test.js
@@ -80,7 +80,7 @@ function fieldWithInputArg(inputArg) {
     type: GraphQLString,
     args: { input: inputArg },
     resolve(_, args) {
-      if (args.hasOwnProperty('input')) {
+      if ('input' in args) {
         return inspect(args.input);
       }
     },
diff --git a/src/language/parser.js b/src/language/parser.js
index aa0bf952bf..491d400743 100644
--- a/src/language/parser.js
+++ b/src/language/parser.js
@@ -1424,7 +1424,7 @@ function parseDirectiveLocations(lexer: Lexer<*>): Array<NameNode> {
 function parseDirectiveLocation(lexer: Lexer<*>): NameNode {
   const start = lexer.token;
   const name = parseName(lexer);
-  if (DirectiveLocation.hasOwnProperty(name.value)) {
+  if (DirectiveLocation[name.value] !== undefined) {
     return name;
   }
   throw unexpected(lexer, start);
diff --git a/src/type/definition.js b/src/type/definition.js
index 43873407ba..6eab19ce00 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -759,7 +759,7 @@ function defineFieldMap<TSource, TContext>(
       `${config.name}.${fieldName} field config must be an object`,
     );
     invariant(
-      !fieldConfig.hasOwnProperty('isDeprecated'),
+      !('isDeprecated' in fieldConfig),
       `${config.name}.${fieldName} should provide "deprecationReason" ` +
         'instead of "isDeprecated".',
     );
@@ -1253,7 +1253,7 @@ function defineEnumValues(
         `representing an internal value but got: ${inspect(value)}.`,
     );
     invariant(
-      !value.hasOwnProperty('isDeprecated'),
+      !('isDeprecated' in value),
       `${type.name}.${valueName} should provide "deprecationReason" instead ` +
         'of "isDeprecated".',
     );
@@ -1263,7 +1263,7 @@ function defineEnumValues(
       isDeprecated: Boolean(value.deprecationReason),
       deprecationReason: value.deprecationReason,
       astNode: value.astNode,
-      value: value.hasOwnProperty('value') ? value.value : valueName,
+      value: 'value' in value ? value.value : valueName,
     };
   });
 }
@@ -1379,7 +1379,7 @@ function defineInputFieldMap(
   );
   return mapValue(fieldMap, (fieldConfig, fieldName) => {
     invariant(
-      !fieldConfig.hasOwnProperty('resolve'),
+      !('resolve' in fieldConfig),
       `${config.name}.${fieldName} field has a resolve property, but ` +
         'Input Types cannot define resolvers.',
     );