Flow: Object.assign({}, objOfType) causes `property not found in object literal` error

[kmiyashiro]

flow 0.25
node 6.1.0

Is this expected behavior?

type MyType = {
  optional?: string
}

const test = function test(opts?: MyType = {}) {
  return Object.assign({ hi: 1 }, opts) // property `optional` of object type Property not found in object literal
}

[ericqianli]

I'm having the same issue. It seems when doing Object.assign flow enforces all source fields to exist in the target, which makes no sense to me. See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

[ilkerguller]

+1 😢

[SimenB]

Still present in 0.30

[jamiebuilds]

Strange, a work around for now seems to be:

- return Object.assign({ hi: 1 }, opts)
+ return Object.assign({}, { hi: 1 }, opts)

[SimenB]

@thejameskyle That makes it pass, thanks!

[aweary]

We recently ran into this with React, where we use Object.assign on the prototype of an internal component to extend it from internal component.

Object.assign(ReactTestRenderer.prototype, ReactMultiChild);

[mockdeep]

Not sure if this is the same issue, but running into something similar with Object.assign. Example code:

// @flow
var myObj = { foo: 'blah' }

Object.assign(myObj, { wat: {} });

myObj.wat.bloo = 'blah';

console.log(myObj);

produces the following warning:

flowtest.js:6
  6: myObj.wat.bloo = 'blah';
           ^^^ property `wat`. Property not found in
  6: myObj.wat.bloo = 'blah';
     ^^^^^ object literal

[mockdeep]

Or is this intended behavior around object sealing?

[jamiebuilds]

Yup, this does have to do with object sealing, it's not really a bug.

There's a difference between empty objects and objects with one or more properties.

const obj1 = {}; // unsealed

obj1.foo = 1;       // Works!
obj1.bar = true;    // Works!
obj1.baz = 'three'; // Works!

const obj2 = { foo: 1 }; // sealed

obj2.bar = true;    // Error: Property not found in object literal
obj2.baz = 'three'; // Error: Property not found in object literal

[[Try](https://flowtype.org/try/#0PTAEAEDMBsHsHcBQiDGsB2BnALqWAjAKwEZQBeUAbwF8BuUEUAVywFMBDaVgE2QJIB0kWLHKhi9UFOmMA6rABOAa0wBCRP2ID87BWOwKmrSVLmKV6zdvYAvMQHJsACwWtW9+meVrkaLLn4AJjFKUGFYAC5xUDoGMEwOLl4NIkDrPQoDIxM40ABRBQVFKIAFIoAHVgVsAE9QdFhcYRZuUABLdDwiVhRcaDbsKs4UwjSdOwpHFzcPXIKihVKKqtr6xrDYFvbO-h6+gaHoRCA)]

Closing as this is not actually a bug.

[kmiyashiro]

There's still an issue with the result of Object.assign in this case:

type MyObj = {|
  one: number,
  two: number
|}
var x = { two: 2 }
var a: MyObj = Object.assign({}, { one: 1 }, x)
// object literal Inexact type is incompatible with exact type exact type: object type

[nmn]

@kmiyashiro That's a bug with exact types that's a known bug and mentioned in another issue.

[FezVrasta]

Any way to extend this?

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjOA7AzgFzAFsBLADxOzAF4wAKAJzjnwC4wB5AIwCsBTDPgCUNAHydeA-ADoAhrlwkA5tkbN8AGjABvMF1kN2dEdXEByfQC8zYAL5CA3Ogwx5uMADFmO1GD9gsPHwGAFdBOAZ6EW1ffzjSClV8AAsSXEdYv1t0OPw+AmMfOLjA3DgYPml4JToUtOl9BmMMuOzsoA