TypeScript function combines the input and output of another function

Question

I would like to type the following function:

carryOver = <T extends Record<string, any>>(fun: (obj: T) => Record<string, any>) => (obj: T) => Object.assign({}, obj, fun(obj))

Such that I can do e.g.:

const makeGreeting = ({name, title}) => ({greeting: `Hello ${title} ${name}`})
const person = {name: "Thomas", title: "Dr."}
const personWithLabel = carryOver(makeGreeting)(person) 
// { name: "Thomas", title: "Dr.", greeting: "Hello Dr. Thomas" }

I.e. carryOver() accepts a function that takes in an object A, destructures it, and returns a new object B, with properties computed from the destructured properties of A. carryOver() then combines the objects together, so we end up with augmented object with properties from both A and B.

How do I type carryOver() so that the type of the output has both the props of A and B?

Does this approach meet your needs? If so I'll write up an answer explaining; if not, what am I misssing? — jcalz
– jcalz, Commented Jan 19, 2023 at 4:02

Robby Cornelissen · Accepted Answer · 2023-01-19 04:08:33Z

1

Here's my implementation:

const carryOver = <A extends object, B extends object>(a: A, fn: (a: A) => B)
    => Object.assign({}, a, fn(a));

const makeGreeting = (a: { name: string, title: string })
    => ({greeting: `Hello ${a.name} ${a.title}`});
const person = { name: "Thomas", title: "Dr." };
const personWithLabel = carryOver(person, makeGreeting);

The return type is correctly inferred from the carryOver() function as {} & A & B.

answered Jan 19, 2023 at 4:08

Robby Cornelissen

98.6k23 gold badges154 silver badges180 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Adam B. Over a year ago

Oh yep, type inference works perfectly for this, thanks that makes things a lot easier!

Collectives™ on Stack Overflow

TypeScript function combines the input and output of another function

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related