Prefer using exact object types JS-0494
Type check
Minor
Autofix It is recommended to use exact object types wherever possible as it improves type checking. It is considered safe to pass an object with extra properties where a normal object type is expected in Flow. So when we add type for an object, for example:
let obj: { prop: string } = { prop : "someValue" }
This object obj can even be assigned with more extra properties like this.
obj = { prop : "someValue", prop2: "someOtherValule" } // fine, no typechecking errors
Flow wont throw any typechecking error here. This is because { prop : "someValue", prop2: "someOtherValule" } is subtype of { prop : "someValue" }.
Here we can see that this is not a proper and accurate type checking as we might need to know what all properties/methods are present in an object. This is why exact object types are preferred as exact object types disable this subtyping and disallow extra properties.
let obj: { | prop: string | } = { prop : "someValue" } // ok
obj = { prop : "someValue", prop2: "someOtherValule" } // Error.
Bad Practice
type obj = {};
type obj = { city: string };
type obj = Array<{city: string}>;
(prop: Array<{city: string}>) => {};
Recommended
type obj = {| |};
type obj = {| city: string |};
type obj = { [key: string]: string };
type obj = number;
type obj = {| city: {| pin: string |} |};