Flow: Providing Flow typing for function argument when using default object values

[jmarceli]

I'm not sure but I think that this might be a Flow bug. I'm unable to add Flow typing when using Object.assign to provide default values for function arguments. Here is a SO question: https://stackoverflow.com/questions/51422411/flow-default-values-type-checking/51431098

And here is a direct link to reproduce this problem on flow.org/try: https://flow.org/try...

Any help will be appreciated.

[wchargin]

Your problem can be reduced to a simpler example—with intersection
types, optional and non-optional properties don’t mix well:

// @flow
type ConfigIn = {someValue?: number};
type Config = ConfigIn & {someValue: number};
const c: Config = {someValue: 1000};  // error!

(Potentially related: #5929.)

Using the type-spread operator (...) instead fixes this, and fixes
the downstream problem in your example:

// @flow
type ConfigIn = {someValue?: number};
type Config = {...ConfigIn, someValue: number};

function test(config: ConfigIn): number {
  const myConfig: Config = Object.assign(
    {},
    {someValue: 1000},
    config
  );
  return otherFunction(myConfig.someValue);
}

function otherFunction(input: number): number {
  return 123;
}

Frankly, intersection types seem to break more often than they work.
I avoid them altogether.

[jmarceli]

Thanks, this is it.

What is more, for anyone interested, it is sometimes good to use Exact object types to avoid nested properties problem example and solution