Implement TypeScript to include type annotations on functions' parameters using destructuring and the rest syntax

Issues with Typing in Typescript

Trying to learn typing in Typescript has presented some difficulties for me:

I am struggling to convert this code into a strongly-typed format using Typescript.

const omit = (prop: P, { [prop]: _, ...rest}) => rest;

  • The main issue lies in adding typing for the destructured object inside the second parameter.

My Attempt at Solving the Problem

This is my attempted solution, but it does not seem to be functioning correctly:

const omit = <P = string, R>(prop: P, { [prop]: _, ...rest } : {[prop: string], rest: R }): R => rest;

const omit = <P = string, O, R = Omit<O,P>>(prop: P, { [prop]: _, ...rest } : {[prop: string]: O[P], rest: R }): R => rest;

const omit = <P = string, R>(prop: P, { [prop]: _, ...rest } : {[prop]: P, rest: R }): R => rest;

Answer №1

Creating this code is quite simple. Your approach with generics is correct, but you are attempting to define the type of the second parameter based on how it will be used to calculate the function's result, which is not the right way to proceed.

Instead, make the second parameter accept any type by declaring an unconstrained generic, and specify that the first parameter should be restricted to any property key of the type the generic is instantiated with - essentially, whatever type the second parameter represents.

const omit = <O, P extends keyof O>(prop: P, { [prop]: _, ...rest }: O): Omit<O, P> => rest;
const o = { a : 1, b: 2 };
const r = omit('a', o);

Link to Playground

It is important to note that the return type of the function will be correctly inferred as Omit<O, P>, so there is no need to explicitly specify it.

const omit = <O, P extends keyof O>(prop: P, { [prop]: _, ...rest }: O) => rest;

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