fix: resolve issue with conversion string to simple-json (typeorm#4476)

Closes: typeorm#4440

Author: abhijeet1403

src/util/DateUtils.ts

@@ -170,7 +170,12 @@ export class DateUtils {
     }
 
     static stringToSimpleJson(value: any) {
-        return typeof value === "string" ? JSON.parse(value) : value;
+        try {
+            const simpleJSON = JSON.parse(value); 
+            return (typeof simpleJSON === "object") ? simpleJSON : {};
+       } catch (err) {
+            return {};
+       }
     }
 
     static simpleEnumToString(value: any) {

test/github-issues/4440/entity/Post.ts

@@ -0,0 +1,16 @@
+import { Column } from "../../../../src/decorator/columns/Column";
+import { PrimaryColumn } from "../../../../src/decorator/columns/PrimaryColumn";
+import { Entity } from "../../../../src/decorator/entity/Entity";
+
+@Entity()
+export class Post {
+    @PrimaryColumn()
+    id: number;
+
+    @Column({
+        type: "simple-json",
+        nullable: true
+    })
+    jsonField: any;
+
+}

test/github-issues/4440/issue-4440.ts

@@ -0,0 +1,43 @@
+import "reflect-metadata";
+import { Connection } from "../../../src/connection/Connection";
+import { closeTestingConnections, createTestingConnections, reloadTestingDatabases } from "../../utils/test-utils";
+import { Post } from "./entity/Post";
+
+describe("github issues > #4440 simple-json column type throws error for string with no value", () => {
+
+    let connections: Connection[];
+    before(async () => {
+        connections = await createTestingConnections({
+            entities: [Post],
+            schemaCreate: true,
+            dropSchema: true
+        });
+    });
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("should correctly add retrieve simple-json field with no value", () =>
+    Promise.all(connections.map(async (connection) => {
+        const repo = connection.getRepository(Post);
+        const post = new Post();
+        post.id = 1;
+        post.jsonField = "";
+        await repo.save(post);
+        const postFound = await repo.findOne(1);
+        postFound!.id.should.eql(1);
+        postFound!.jsonField.should.eql({});
+    })));
+
+    it("should correctly add retrieve simple-json field with some value", () =>
+    Promise.all(connections.map(async (connection) => {   
+        const repo = connection.getRepository(Post);
+        const post = new Post();
+        post.id = 1;
+        post.jsonField = {"key": "value"};
+        await repo.save(post);
+        const postFound = await repo.findOne(1);
+        postFound!.id.should.eql(1);
+        postFound!.jsonField.should.eql({"key": "value"});
+    })));
+
+});