Preserve @deprecated when reason = null

src/utilities/__tests__/buildClientSchema-test.ts

@@ -382,15 +382,15 @@ describe('Type System: build schema from introspection', () => {
         name: 'VEGETABLES',
         description: 'Foods that are vegetables.',
         value: 'VEGETABLES',
-        deprecationReason: null,
+        deprecationReason: undefined,
         extensions: {},
         astNode: undefined,
       },
       {
         name: 'FRUITS',
         description: null,
         value: 'FRUITS',
-        deprecationReason: null,
+        deprecationReason: undefined,
         extensions: {},
         astNode: undefined,
       },

src/utilities/__tests__/printSchema-test.ts

@@ -591,6 +591,15 @@ describe('Type System Printer', () => {
     `);
   });
 
+  it('Prints deprecated directives with explicitly `null` reason', () => {
+    const SDL = dedent`
+      type Query {
+        someField: String @deprecated(reason: null)
+      }`;
+    const schema = buildSchema(SDL);
+    expectPrintedSchema(schema).to.equal(SDL);
+  });
+
   it('Prints custom directives', () => {
     const SimpleDirective = new GraphQLDirective({
       name: 'simpleDirective',

src/utilities/buildClientSchema.ts

@@ -292,7 +292,9 @@ export function buildClientSchema(
         (valueIntrospection) => valueIntrospection.name,
         (valueIntrospection) => ({
           description: valueIntrospection.description,
-          deprecationReason: valueIntrospection.deprecationReason,
+          deprecationReason: valueIntrospection.isDeprecated
+            ? valueIntrospection.deprecationReason
+            : undefined,
         }),
       ),
     });
@@ -350,7 +352,9 @@ export function buildClientSchema(
 
     return {
       description: fieldIntrospection.description,
-      deprecationReason: fieldIntrospection.deprecationReason,
+      deprecationReason: fieldIntrospection.isDeprecated
+        ? fieldIntrospection.deprecationReason
+        : undefined,
       type,
       args: buildInputValueDefMap(fieldIntrospection.args),
     };
@@ -383,7 +387,9 @@ export function buildClientSchema(
       description: inputValueIntrospection.description,
       type,
       defaultValue,
-      deprecationReason: inputValueIntrospection.deprecationReason,
+      deprecationReason: inputValueIntrospection.isDeprecated
+        ? inputValueIntrospection.deprecationReason
+        : undefined,
     };
   }
 

src/utilities/printSchema.ts

@@ -282,11 +282,15 @@ export function printDirective(directive: GraphQLDirective): string {
 }
 
 function printDeprecated(reason: Maybe<string>): string {
-  if (reason == null) {
+  if (reason === undefined) {
     return '';
   }
   if (reason !== DEFAULT_DEPRECATION_REASON) {
-    const astValue = print({ kind: Kind.STRING, value: reason });
+    const astValue = print(
+      reason === null
+        ? { kind: Kind.NULL }
+        : { kind: Kind.STRING, value: reason },
+    );
     return ` @deprecated(reason: ${astValue})`;
   }
   return ' @deprecated';