PudgePacket PudgePacket - 13 days ago 6
Javascript Question

Flow doesn't complain about incorrect type

In this piece of code, flow is not complaining about the value

dog
being set on the state. It seems to be ignoring the
NamespaceData
definition. I've set up the types so it should complain. I'm running on nuclide and flow is working properly for everything else.

All of the properties of
action
such as
namespace
,
project
,
collection
are strings.

// @flow

import { NAMESPACE_SET } from '../actions/NamespaceActions'

type NamespaceData = {
project: string,
collection: string,
}

type NamespaceState = {
[namespace: string]: NamespaceData,
}
const initialState: NamespaceState = {}

function namespaceReducer(state: NamespaceState = initialState, action: Object): NamespaceState {
switch (action) {
case NAMESPACE_SET: {
return {
...state,
[action.namespace]: {
project: action.project,
collection: action.collection,
dog: 1,
}
}
}
}
return state
}

export default namespaceReducer

Answer

Flow is not strict about unknown properties in objects by default, e.g.

// @flow

type Thing = {
  known: string;
};

var obj: Thing = {
  known: 'hi',
  unknown: 4,
};

typechecks fine even though unknown is not in the type.

Flow 0.32 includes

  • New syntax for exact object types: use {| and |} instead of { and }. Where {x: string} contains at least the property x, {| x: string |} contains ONLY the property x.

In your example you'd want exact object syntax with:

type NamespaceData = {|
  project: string,
  collection: string,
|};
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