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);
    }));
});

Only the first and last test pass, namely:

• undefined params are replaced with the default params value { foo: "Default", bar: 42}
• fully defined params are of course processed correctly

In all the in-between cases, the default value is undefined since when we provide an object, the entire params is taken as provided and no default values are being substituted.

Kind of reasonable, but sometimes tricky to see immediately.

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