Typecasting Object assignment within Array.forEach

Hi,

This might be a silly question but...

I need to reset some Object values and I decided to use a forEach loop. This Object values are defined as it follows:

Basically, I've declared this Object type. It's important the Object contains at least 2 attributes with different types. For example, a number and a string:

type FooType = {|
  a: number,
  b: string
|}

And I have written an Object of the same type that will be used to set/reset the initial values in another one of the same type.

const INITIAL_FOO: FooType = {
  a: 0,
  b: ''
}
const foo: FooType = {
  a: 1,
  b: 'qwerty'
}

Therefore, when I use the Array.forEach for an assignment I get the following flow error:

Object.keys(foo).forEach((key) => {
  foo[key] = (INITIAL_FOO[key]: any); // This works
})
Object.keys(foo).forEach((key) => {
  foo[key] = INITIAL_FOO[key];        // This fails
})

18:   foo[key] = INITIAL_FOO[key];        // This fails
                 ^ Cannot assign `INITIAL_FOO[key]` to `foo[key]` because number [1] is incompatible with string [2]. [incompatible-type]
References:
3:   a: number,
        ^ [1]
4:   b: string        ^ [2]
18:   foo[key] = INITIAL_FOO[key];        // This fails
                 ^ Cannot assign `INITIAL_FOO[key]` to `foo[key]` because string [1] is incompatible with number [2]. [incompatible-type]
References:
4:   b: string        ^ [1]
3:   a: number,
        ^ [2]

I guess that Flow is not able to detect that the key used to read from INITIAL_FOO and the key used to write in foo are the same on every loop.

Is there any way to fix this and keep the forEach loop?

Here is a link to test this example: https://flow.org/try

Thanks,

flowjs / flow.js

Typecasting Object assignment within Array.forEach #377