December 05, 2018 by Artur

Hello, this is starting the article for advanced typescript tutorial series. Today I’ll cover basic usage of:

infer

For me, it was hard to understand at the beginning what I can really do with infer . Let’s start with a really basic example.

type FlattenIfArray < T > = T extends ( infer R ) [ ] ? R : T

So let’s analyze this code:

We check if our generic Type is the array If it is array extract the real type from it If it does not leave it as is

Still, it is not clear what infer is doing, so let’s proceed with another example

type Unpromisify < T > = T extends Promise < infer R > ? R : T

This one looks more clear as it doesn’t have parenthesis:

We check if type extends Promise If it does we extract the type from the promise If it does not leave it as is

See? If you use extends only just to check if the type is a promise you would use

type Unpromisify < T > = T extends Promise < any > ? T : never

And in infer instead of any keyword, you infer the value from type. Let’s try with more advanced types then:

type FuncWithOneObjectArgument < P extends { [ x : string ] : any } , R > = ( props : P ) => R ; type DestructuredArgsOfFunction < F extends FuncWithOneObjectArgument < any , any > > = F extends FuncWithOneObjectArgument < infer P , any > ? P : never ; const myFunction = ( props : { x : number ; y : number } ) : string => { return "OK" ; } ; const props : DestructuredArgsOfFunction < typeof myFunction > = { x : 1 , y : 2 } ;

Intellisense for props works like this:

You can make use of it inferring React Component props for example or another function that uses destructured params.