FlowType注释函数返回类型

Question

I need to annotate in FlowType a function which return an object, I see I have several options:

A) Annotation on object exported and function

const getForecastHourly:ActionType = (query:number):ActionType => ...

B) Annotation on function only:

const getForecastHourly = (query:number):ActionType => ...

C) Annotation on object exported only:

const getForecastHourly:ActionType  = (query:number) => ...

In my code I am using version A), but I would like to know if B or C could be equivalent and which version is suggestible and why.

 // @flow import {ActionType} from '../../types' import 'isomorphic-fetch' import * as api from '../../app/api' import * as types from './forecastHourlyActionTypes' const getForecastHourly:ActionType = (query:number):ActionType => ({ type: types.GET_FORECAST_HOURLY, payload: new Promise((resolve, reject) => { fetch(api.forecast(query)).then(response => { resolve(response.json()) }) }) }) const setForecastHourlyActiveReportType:ActionType = (type:string):ActionType => ({ type: types.SET_FORECAST_HOURLY_ACTIVE_REPORT_TYPE, payload: type }) export { getForecastHourly, setForecastHourlyActiveReportType } 

 export type ActionType ={ +type:string, +payload: Object } 

Answer 1

What I do is only annotate the function itself like

const getForecastHourly = (query: number): ActionType => ({ /* - */ });

Because flow knows about const , it know the value can't change and will figure the type out itself.

On the other hand if you'd use a let , then I'd annotate the variable itself also, so flow can check on reassigning the value if it has the correct type.