A test demonstrating Typescript does not support "deep" default params, i.e. the default params do not extend to object properties.

typescript-default-params.spec.ts

interface Params {
  foo?: string;
  bar?: number;
}

function doStuff(params: Params = { foo: "Default", bar: 42}): any[] {
  return [params.bar, params.foo];
}

describe("default params", () => {

    const examples: ([Params, any[]])[] = [
      [undefined, [42, "Default"]],
      [{}, [42, "Default"]],
      [{foo: "FOO"}, [42, "FOO"]],
      [{bar: 13}, [13, "Default"]],
      [{foo: "Barf", bar: 17}, [17, "Barf"]],
    ];

    examples.forEach(example => it(JSON.stringify(example), () => {
      const [params, result] = example;
      expect(doStuff(params)).toEqual(result);
    }));
});

Here's the official stuff: https://www.typescriptlang.org/docs/handbook/functions.html#optional-and-default-parameters