Is there any way to target the plain JavaScript object type in TypeScript?

Tested in TypeScript 3.7.2:

For a flat plain object, you can do:

type Primitive =
  | bigint
  | boolean
  | null
  | number
  | string
  | symbol
  | undefined;

type PlainObject = Record<string, Primitive>;

class MyClass {
  //
}

const obj1: PlainObject = { a: 1 }; // Works
const obj2: PlainObject = new MyClass(); // Error

For a nested plain object:

type Primitive =
  | bigint
  | boolean
  | null
  | number
  | string
  | symbol
  | undefined;

type JSONValue = Primitive | JSONObject | JSONArray;

interface JSONObject {
  [key: string]: JSONValue;
}

interface JSONArray extends Array<JSONValue> { }

const obj3: JSONObject = { a: 1 }; // Works
const obj4: JSONObject = new MyClass(); // Error

const obj5: JSONObject = { a: { b: 1 } }; // Works
const obj6: JSONObject = { a: { b: { c: 1 } } }; // Works
const obj7: JSONObject = { a: { b: { c: { d: 1 } } } }; // Works

Code is an adaptation from https://github.com/microsoft/TypeScript/issues/3496#issuecomment-128553540

In my code I have something similiar to what you're asking:

export type PlainObject = { [name: string]: any }
export type PlainObjectOf<T> = { [name: string]: T }

And I also have a type guard for that:

export function isPlainObject(obj: any): obj is PlainObject {
    return obj && obj.constructor === Object || false;
}

---

Edit

Ok, I understand what you're looking for, but unfortunately that is not possible.
If i understand you correctly then this is what you're after:

type PlainObject = {
    constructor: ObjectConstructor;
    [name: string]: any
}

The problem is that in 'lib.d.ts' Object is defined like so:

interface Object {
    /** The initial value of Object.prototype.constructor is the standard built-in Object constructor. */
    constructor: Function;

    ...
}

And then this:

let o: PlainObject = { key: "value" };

Results with an error:

Type '{ key: string; }' is not assignable to type 'PlainObject'.
  Types of property 'constructor' are incompatible.
    Type 'Function' is not assignable to type 'ObjectConstructor'.
      Property 'getPrototypeOf' is missing in type 'Function'.