Set default objects with Object.assign or destructuring

Bad:

type MenuConfig = { title?: string; body?: string; buttonText?: string; cancellable?: boolean };

function createMenu(config: MenuConfig) {
  config.title = config.title || 'Foo';
  config.body = config.body || 'Bar';
  config.buttonText = config.buttonText || 'Baz';
  config.cancellable = config.cancellable !== undefined ? config.cancellable : true;

  // ...
}

createMenu({ body: 'Bar' });

Good:

type MenuConfig = { title?: string; body?: string; buttonText?: string; cancellable?: boolean };

function createMenu(config: MenuConfig) {
  const menuConfig = Object.assign(
    {
      title: 'Foo',
      body: 'Bar',
      buttonText: 'Baz',
      cancellable: true
    },
    config
  );

  // ...
}

createMenu({ body: 'Bar' });

Or, you could use the spread operator:

function createMenu(config: MenuConfig) {
  const menuConfig = {
    title: 'Foo',
    body: 'Bar',
    buttonText: 'Baz',
    cancellable: true,
    ...config
  };

  // ...
}

The spread operator and Object.assign() are very similar.
The main difference is that spreading defines new properties, while Object.assign() sets them. More detailed, the difference is explained in this thread.

Alternatively, you can use destructuring with default values:

type MenuConfig = { title?: string; body?: string; buttonText?: string; cancellable?: boolean };

function createMenu({ title = 'Foo', body = 'Bar', buttonText = 'Baz', cancellable = true }: MenuConfig) {
  // ...
}

createMenu({ body: 'Bar' });

To avoid any side effects and unexpected behavior by passing in explicitly the undefined or null value, you can tell the TypeScript compiler to not allow it.
See –strictNullChecks option in TypeScript.