I am having trouble figuring out how to type useState function since it returns a tuple. In essence, I have to provide null as initial value for email i.e. lets assume I can't use empty string here.
I then have setEmail function to update this state value, which takes in email as string.
ideally I would like to type my useState so it expects email to be either string or null if possible. At the moment it inherits it as only null
import * as React from "react";
const { useState } = React;
function Example() {
const [state, setState] = useState({ email: null, password: null });
function setEmail(email: string) {
setState(prevState => ({ ...prevState, email }))
}
return <p>{state.email}</p>
}
Following error is returned for setEmail function since string in function argument is not valid type for null specified in useState()
[ts]
Argument of type '(prevState: { email: null; password: null; }) => { email: string; password: null; }' is not assignable to parameter of type 'SetStateAction<{ email: null; password: null; }>'.
Type '(prevState: { email: null; password: null; }) => { email: string; password: null; }' is not assignable to type '(prevState: { email: null; password: null; }) => { email: null; password: null; }'.
Type '{ email: string; password: null; }' is not assignable to type '{ email: null; password: null; }'.
Types of property 'email' are incompatible.
Type 'string' is not assignable to type 'null'. [2345]
(parameter) prevState: {
email: null;
password: null;
}
Currently, the TypeScript compiler thinks the type of email and password are null (and no other value). You can resolve this by providing an explicit type parameter to the useState call so that the types of email and password are known to be string or null.
const { useState } = React;
function Example() {
const [state, setState] = useState<{email: null | string, password: null | string}>({ email: null, password: null });
function setEmail(email: string) {
setState(prevState => ({ ...prevState, email }))
}
return <p>{state.email}</p>
}
This is addressed in a few spots already:
https://dev.to/busypeoples/notes-on-typescript-react-hooks-28j2
https://codewithstyle.info/Using-React-useState-hook-with-TypeScript/
TLDR: pass a type argument to setState when you have an empty initial state
eg:
const [email, setEmail] = useState<string>();
you can use TS mapped types to improve readability and prefer undefined over null values
const { useState } = React;
function Example() {
const [state, setState] = useState<Partial<{email: string, password: string}>>();
function setEmail(email: string) {
setState(prevState => ({ ...prevState, email }))
}
return <p>{state.email | ""}</p>
}