Do NOT run yarn install manually - everything is ready to go!

The purpose of this repository is to understand implications of switching from explicit exact Flow types ({||}) to implicit exact Flow types ({}) for our published NPM libraries.

Test 1 (the default exact_by_default=false)

Change node_modules/@adeira/test/index.js to:

// @flow

export type A = {| x: string |} // exact
export type B = { x: string, ... } // inexact
export type C = { x: string } // inexact

And .flowconfig to:

[options]
exact_by_default=false

Result:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:5:14

Cannot assign object literal to a because property y is missing in A [1] but exists in object literal [2].
[prop-missing]

        2│
        3│ import type {A, B, C} from '@adeira/test'
        4│
 [1][2] 5│ const a: A = {x:"", y:""}
        6│ const b: B = {x:"", y:""}
        7│ const c: C = {x:"", y:""}
        8│

Test 2 (exact_by_default=true)

Change node_modules/@adeira/test/index.js to:

// @flow

export type A = {| x: string |} // exact
export type B = { x: string, ... } // inexact
export type C = { x: string } // exact

And .flowconfig to:

[options]
exact_by_default=true

Result:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:5:14

Cannot assign object literal to a because property y is missing in A [1] but exists in object literal [2].
[prop-missing]

        2│
        3│ import type {A, B, C} from '@adeira/test'
        4│
 [1][2] 5│ const a: A = {x:"", y:""}
        6│ const b: B = {x:"", y:""}
        7│ const c: C = {x:"", y:""}
        8│


Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:7:14

Cannot assign object literal to c because property y is missing in C [1] but exists in object literal [2].
[prop-missing]

         4│
         5│ const a: A = {x:"", y:""}
         6│ const b: B = {x:"", y:""}
 [1][2]  7│ const c: C = {x:"", y:""}
         8│

Test 3 (converted NPM with the default exact_by_default=false)

Change node_modules/@adeira/test/index.js to:

// @flow

export type A = { x: string } // inexact (CHANGED)
export type B = { x: string, ... } // inexact
export type C = { x: string } // inexact

And .flowconfig to:

[options]
exact_by_default=false

Result:

No errors!

Test 4 (converted NPM with exact_by_default=true)

Change node_modules/@adeira/test/index.js to:

// @flow

export type A = { x: string } // exact (CHANGED)
export type B = { x: string, ... } // inexact
export type C = { x: string } // exact

And .flowconfig to:

[options]
exact_by_default=true

Result:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:5:14

Cannot assign object literal to a because property y is missing in A [1] but exists in object literal [2].
[prop-missing]

        2│
        3│ import type {A, B, C} from '@adeira/test'
        4│
 [1][2] 5│ const a: A = {x:"", y:""}
        6│ const b: B = {x:"", y:""}
        7│ const c: C = {x:"", y:""}
        8│


Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ index.js:7:14

Cannot assign object literal to c because property y is missing in C [1] but exists in object literal [2].
[prop-missing]

         4│
         5│ const a: A = {x:"", y:""}
         6│ const b: B = {x:"", y:""}
 [1][2]  7│ const c: C = {x:"", y:""}
         8│

Conclusion

Users with exact_by_default=false (legacy default)

Change from explicit exact objects ({||}) to implicit exact objects ({}) can have unintended effects. Such users should convert their codebase as soon as possible:

• first to explicit exact objects ({||}) and explicit inexact objects ({...})
• second from explicit exact objects ({||}) to implicit exact objects ({}) while keeping explicit inexact objects

Users with exact_by_default=true (the old new)

There is no change and everything should be good to go. ✅