TL;DR: Use bracket notation to Access private properties

expect(inst["_somePrivateProp"]).toBe("foo"); // OK

Disclaimer

This is not a lengthy discussion. But a small reference type post. There are other (lengthy) posts that go into the details. I like it short.

Introduction

There is some ongoing debate about testing private members (methods, fields, properties, ...). In my opinion there are occasions where it is useful to do that.

JavaScript/TypeScript

JavaScript/TypeScript does not require you to do that (at least for TypeScript private members, not for ES private (#) class fields).

// mod.ts
export class SomeClass {
    private _somePrivateProp = "foo";
}

JavaScript does not care if your member is marked private by TypeScript.
The TypeScript compiler enforces that you don't access private members from outside the class using dot notation (inst.prop).
But it is possible to access the property using bracket notation (inst["prop"]).

// mod.test.ts
import {SomeClass} from "./mod.ts";
test("privatePropIsFoo", () => {
    const inst = new SomeClass();
    expect(inst._somePrivateProp).toBe("foo"); // TS2341: Property '_somePrivateProp' is private and only accessible within class 'SomeClass'.

    expect(inst["_somePrivateProp"]).toBe("foo"); // OK
})

This is great because refactoring tools still work. And it saves a lot of redundant code.

A caveat

jest.spy(inst, "_privateMethod") will not work, because TypeScript enforces that only public methods are spied on. The simplest workaround is to cast inst as any. Until there is a Utility Type that treats all members as public (as already suggested).