fix(ast): correctly mark fields as nullable when not required

Creates a new type alias `Nullable<T>` which will make a given field nullable
if that field is not required by the schema.

This more accurately models the allowed types for GraphQL since GraphQL will
accept `null` as a valid value when a field is marked as such. Prior to this change,
`null` was not allowed as a value in TypeScript since the generated type definition did not allow it.

Closes #1128

lib/graphql-ast.explorer.ts

@@ -109,6 +109,17 @@ export class GraphQLAstExplorer {
       ? `${DEFINITIONS_FILE_HEADER}\n${options.additionalHeader}\n\n`
       : DEFINITIONS_FILE_HEADER;
     tsFile.insertText(0, header);
+    tsFile.addTypeAlias({
+      name: 'Nullable',
+      isExported: false,
+      type: 'T | null',
+      typeParameters: [
+        {
+          name: 'T',
+        },
+      ],
+    });
+
     return tsFile;
   }
 
@@ -263,6 +274,7 @@ export class GraphQLAstExplorer {
     }
 
     const { name: type, required } = this.getFieldTypeDefinition(item.type);
+
     if (!this.isRoot(parentRef.getName())) {
       (parentRef as InterfaceDeclaration).addProperty({
         name: propertyName,
@@ -271,6 +283,7 @@ export class GraphQLAstExplorer {
       });
       return;
     }
+
     if (options.skipResolverArgs) {
       (parentRef as ClassDeclaration).addProperty({
         name: propertyName,
@@ -281,9 +294,7 @@ export class GraphQLAstExplorer {
       (parentRef as ClassDeclaration).addMethod({
         isAbstract: mode === 'class',
         name: propertyName,
-        returnType: `${this.addSymbolIfRoot(
-          type,
-        )} | Promise<${this.addSymbolIfRoot(type)}>`,
+        returnType: `${type} | Promise<${type}>`,
         parameters: this.getFunctionParameters(
           (item as FieldDefinitionNode).arguments,
         ),
@@ -292,28 +303,37 @@ export class GraphQLAstExplorer {
   }
 
   getFieldTypeDefinition(
-    type: TypeNode,
+    typeNode: TypeNode,
   ): {
     name: string;
     required: boolean;
   } {
-    const { required, type: nestedType } = this.getNestedType(type);
-    type = nestedType;
+    const { required, type } = this.getNestedType(typeNode);
 
     const isArray = type.kind === 'ListType';
     if (isArray) {
-      const { type: nestedType } = this.getNestedType(get(type, 'type'));
-      type = nestedType;
+      const {
+        type: arrayType,
+        required: arrayTypeRequired,
+      } = this.getNestedType(get(type, 'type'));
+
+      const typeName = this.addSymbolIfRoot(get(arrayType, 'name.value'));
+      const name = arrayTypeRequired
+        ? this.getType(typeName)
+        : `Nullable<${this.getType(typeName)}>`;
 
-      const typeName = get(type, 'name.value');
       return {
-        name: this.getType(typeName) + '[]',
+        name: required ? name + '[]' : `Nullable<${name}[]>`,
         required,
       };
     }
-    const typeName = get(type, 'name.value');
+
+    const typeName = this.addSymbolIfRoot(get(type, 'name.value'));
+
     return {
-      name: this.getType(typeName),
+      name: required
+        ? this.getType(typeName)
+        : `Nullable<${this.getType(typeName)}>`,
       required,
     };
   }

tests/generated-definitions/array-property.fixture.ts

@@ -8,7 +8,9 @@
 /* eslint-disable */
 export interface Foo {
     a: string[];
-    b?: string[];
-    c: string[];
-    d?: string[];
+    b?: Nullable<string[]>;
+    c: Nullable<string>[];
+    d?: Nullable<Nullable<string>[]>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/custom-header.fixture.ts

@@ -10,3 +10,4 @@
 /* Put anything here you like */
 
 export type CustomDate = any;
+type Nullable<T> = T | null;

tests/generated-definitions/custom-scalar-default-type.fixture.ts

@@ -7,3 +7,4 @@
 /* tslint:disable */
 /* eslint-disable */
 export type CustomDate = unknown;
+type Nullable<T> = T | null;

tests/generated-definitions/custom-scalar-type-mapping.fixture.ts

@@ -9,3 +9,4 @@
 export type List = string[];
 export type Point = [number, number];
 export type DateTime = Date;
+type Nullable<T> = T | null;

tests/generated-definitions/custom-scalar.fixture.ts

@@ -7,3 +7,4 @@
 /* tslint:disable */
 /* eslint-disable */
 export type CustomDate = any;
+type Nullable<T> = T | null;

tests/generated-definitions/enum.fixture.ts

@@ -11,3 +11,5 @@ export enum Foobar {
     Bar = "Bar",
     Baz = "Baz"
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/federation.fixture.ts

@@ -18,10 +18,12 @@ export class Post {
 }
 
 export abstract class IQuery {
-    abstract getPosts(): Post[] | Promise<Post[]>;
+    abstract getPosts(): Nullable<Nullable<Post>[]> | Promise<Nullable<Nullable<Post>[]>>;
 }
 
 export class User {
     id: string;
-    posts?: Post[];
+    posts?: Nullable<Nullable<Post>[]>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/interface-property.fixture.ts

@@ -12,5 +12,7 @@ export interface Bar {
 
 export interface Foo {
     a: Bar;
-    b?: Bar;
+    b?: Nullable<Bar>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/mutation.fixture.ts

@@ -11,10 +11,12 @@ export interface Cat {
 }
 
 export interface IMutation {
-    createCat(name?: string): Cat | Promise<Cat>;
-    returnsQuery(): IQuery | Promise<IQuery>;
+    createCat(name?: Nullable<string>): Nullable<Cat> | Promise<Nullable<Cat>>;
+    returnsQuery(): Nullable<IQuery> | Promise<Nullable<IQuery>>;
 }
 
 export interface IQuery {
-    query(): number | Promise<number>;
+    query(): Nullable<number> | Promise<Nullable<number>>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/query-skip-args.fixture.ts

@@ -11,5 +11,7 @@ export interface Cat {
 }
 
 export interface IQuery {
-    cat?: Cat;
+    cat?: Nullable<Cat>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/query.fixture.ts

@@ -11,5 +11,7 @@ export interface Cat {
 }
 
 export interface IQuery {
-    cat(id: string): Cat | Promise<Cat>;
+    cat(id: string): Nullable<Cat> | Promise<Nullable<Cat>>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/simple-type.fixture.ts

@@ -9,7 +9,9 @@
 export interface Cat {
     id: number;
     name: string;
-    age?: number;
-    color?: string;
-    weight?: number;
+    age?: Nullable<number>;
+    color?: Nullable<string>;
+    weight?: Nullable<number>;
 }
+
+type Nullable<T> = T | null;

tests/generated-definitions/typename.fixture.ts

@@ -10,7 +10,9 @@ export interface Cat {
     __typename?: 'Cat';
     id: number;
     name: string;
-    age?: number;
-    color?: string;
-    weight?: number;
+    age?: Nullable<number>;
+    color?: Nullable<string>;
+    weight?: Nullable<number>;
 }
+
+type Nullable<T> = T | null;