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Advanced typescript tutorial - infer

December 05, 2018

Hello This is starting article for advanced typescript tutorial series. Today I’ll cover basic usage of


keyword. For me it was hard to understand at the beginning what I can really do with infer. Lets start with really basic example.

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

So lets analyze this code:

  1. We check if our generic Type is array
  2. If it is array extract real type from it
  3. If it is not leave it as is

Still its not clear what infer is doing, so lets 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:

  1. We check if type extends Promise
  2. If it do we extract type from promise
  3. If it is not leave it as is

See? If you use extends only just to check if 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. Lets 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: intellisense.png

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

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Artur Czemiel
Written by Artur Czemiel GraphQL passionate. Code generation guru. Short code lover. Father. CTO. CEO. email me:[email protected]Twitter Shield
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