nico.fyi
Published on

Identity Helper Type in TypeScript

Authors
  • avatar
    Name
    Nico Prananta
    Twitter
    @2co_p

Someone on Twitter asked what the following TypeScript custom generic actually does:

type Identity<T> = T extends object ? { [K in keyof T]: T[K] } : T

Can you figure it out? Let me explain. This TypeScript type, Identity<T>, is a utility type that takes a generic type T and returns a type structurally identical to T. Essentially, it's a no-op for TypeScript's type system, meaning it doesn't modify the type in any significant way. Here's a breakdown:

  • T extends object ? { [K in keyof T]: T[K] } : T uses a conditional type to check if T is an object type.
    • If T is an object (i.e., not a primitive type like string, number, or boolean), it maps over all keys of T (keyof T) and constructs a new type with the same keys and their corresponding types (T[K]). This process recreates the original type T without modification.
    • If T is not an object, it simply returns T as is.

So, what's its actual purpose? One application is to unpack intersection types. Consider the following code:

import { Blog } from 'contentlayer/generated'
type Identity<T> = T extends object ? { [K in keyof T]: T[K] } : T

type BlogWithAdditionalData = Blog & { someProp: string }
const doSomethingWithBlog = async (blog: BlogWithAdditionalData) => {
  console.log(blog)
}

The doSomethingWithBlog function takes an argument of type BlogWithAdditionalData. When I hover over the blog argument or the BlogWithAdditionalData, I see the intersection type:

which isn't very helpful because I can't see the properties of the Blog type. This is where the Identity type comes in handy by unpacking the Blog type and combining it with someProp:

Now, I can see all the properties of BlogWithAdditionalData.

Do you know any other useful applications for the Identity type?


Are you working in a team environment and your pull request process slows your team down? Then you have to grab a copy of my book, Pull Request Best Practices!