TypeScript typeof Function return value

Question

Admit I have a function like this

const createPerson = () => ({ firstName: 'John', lastName: 'Doe' }) 

How can I, without declaring an interface or a type before declaring createPerson, get the return value type?

Something like this:

type Person = typeof createPerson() 

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Example Scenario

I have a Redux container that maps state and dispatch actions to props of a component.

containers/Counter.tsx

import { CounterState } from 'reducers/counter'  // ... Here I also defined MappedState and mapStateToProps  // The interface I would like to remove interface MappedDispatch {   increment: () => any }  // And get the return value type of this function const mapDispatchToProps =   (dispatch: Dispatch<State>): MappedDispatch => ({     increment: () => dispatch(increment)   })  // To export it here instead of MappedDispatch export type MappedProps = MappedState & MappedDispatch export default connect(mapStateToProps, mapDispatchToProps)(Counter) 

components/Counter.tsx

import { MappedProps } from 'containers/Counter'  export default (props: MappedProps) => (   <div>     <h1>Counter</h1>     <p>{props.value}</p>     <button onClick={props.increment}>+</button>   </div> ) 

I want to be able to export the type of mapDispatchToProps without having to create MappedDispatch interface.

I reduced the code here, but it makes me type the same thing two times.

Answer 1

Original Post

TypeScript < 2.8

I created a little library that permits a workaround, until a fully declarative way is added to TypeScript:

https://npmjs.com/package/returnof

Also created an issue on Github, asking for Generic Types Inference, that would permit a fully declarative way to do this:

https://github.com/Microsoft/TypeScript/issues/14400

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Update February 2018

TypeScript 2.8

TypeScript 2.8 introduced a new static type ReturnType which permits to achieve that:

https://github.com/Microsoft/TypeScript/pull/21496

You can now easily get the return type of a function in a fully declarative way:

const createPerson = () => ({   firstName: 'John',   lastName: 'Doe' })  type Person = ReturnType<typeof createPerson>