chore(index.d.ts): add support for lean docs

Re: #8108

index.d.ts

@@ -472,10 +472,10 @@ declare module "mongoose" {
     set(value: any): this;
 
     /** The return value of this method is used in calls to JSON.stringify(doc). */
-    toJSON(options?: ToObjectOptions): any;
+    toJSON(options?: ToObjectOptions): LeanDocument<this>;
 
     /** Converts this document into a plain-old JavaScript object ([POJO](https://masteringjs.io/tutorials/fundamentals/pojo)). */
-    toObject(options?: ToObjectOptions): any;
+    toObject(options?: ToObjectOptions): LeanDocument<this>;
 
     /** Clears the modified state on the specified path. */
     unmarkModified(path: string);
@@ -1480,7 +1480,7 @@ declare module "mongoose" {
     j(val: boolean | null): this;
 
     /** Sets the lean option. */
-    lean(val?: boolean | any): this;
+    lean(val?: boolean | any): Query<LeanDocument<DocType>, DocType>;
 
     /** Specifies the maximum number of documents the query will return. */
     limit(val: number): this;
@@ -1702,7 +1702,9 @@ declare module "mongoose" {
 
   export type UpdateQuery<T> = mongodb.UpdateQuery<T> & mongodb.MatchKeysAndValues<T>;
 
-  export type DocumentDefinition<T> = Omit<T, Exclude<keyof Document, '_id'>>
+  export type DocumentDefinition<T> = Omit<T, Exclude<keyof Document, '_id'>>;
+
+  export type LeanDocument<T> = Omit<T, Exclude<keyof Document, '_id'>>;
 
   class QueryCursor<DocType extends Document> extends stream.Readable {
     /**

test/typescript/leanDocuments.ts

@@ -0,0 +1,23 @@
+import { Schema, model, Document, Types } from 'mongoose';
+
+const schema: Schema = new Schema({ name: { type: 'String' } });
+
+interface ITest extends Document {
+  _id?: Types.ObjectId,
+  name?: string;
+}
+
+const Test = model<ITest>('Test', schema);
+
+void async function main() {
+  const doc: ITest = await Test.findOne();
+
+  const pojo = doc.toObject();
+  await pojo.save();
+
+  const _doc = await Test.findOne().lean();
+  await _doc.save();
+
+  const hydrated = Test.hydrate(_doc);
+  await hydrated.save();
+}();

test/typescript/main.test.js

@@ -119,6 +119,16 @@ describe('typescript syntax', function() {
     }
     assert.equal(errors.length, 0);
   });
+
+  it('lean documents', function() {
+    const errors = runTest('leanDocuments.ts');
+    if (process.env.D && errors.length) {
+      console.log(errors);
+    }
+    assert.equal(errors.length, 2);
+    assert.ok(errors[0].messageText.includes('Property \'save\' does not exist'), errors[0].messageText);
+    assert.ok(errors[1].messageText.includes('Property \'save\' does not exist'), errors[0].messageText);
+  });
 });
 
 function runTest(file) {