Understanding the "Object Literal May Only Specify Known Properties" Error in React + TypeScript

[Lomanu4]

Пожалуйста Авторизируйтесь или Зарегистрируйтесь для просмотра скрытого текста.

Understanding the "Object Literal May Only Specify Known Properties" Error in React + TypeScript


When working with useState and TypeScript in a React project, you might encounter this confusing TypeScript error:

Object literal may only specify known properties, and 'uid' does not exist in type '(prevState: undefined) => undefined'.ts(2353)

Let’s unpack this error, why it happens, and how to properly fix it while demonstrating best practices in state typing using a real-world example.

Problem Overview


Consider the following component:


import { useState } from "react";

export const User = () => {
const [user, setUser] = useState();

const login = () => {
setUser({
uid: 'ABC123',
name: 'Fernando'
});
}

return (
<div className="mt-5">
<h3>User: useState</h3>
<button className="btn btn-outline-primary">Login</button>
</div>
)
}

When you run this, you might get an error like:


Object literal may only specify known properties, and 'uid' does not exist in type '(prevState: undefined) => undefined'.

Why?

Root Cause


The issue is simple: you did not define a type for the user state. TypeScript assumes useState() holds undefined, so when you try to set it with an object, it complains:

“I don’t know what uid or name is supposed to be.”

Solution: Define a Type or Interface


interface User {
uid: string;
name: string;
}

export const UserComponent = () => {
const [user, setUser] = useState<User | null>(null);

const login = () => {
setUser({
uid: 'ABC123',
name: 'Fernando'
});
}

return (
<div className="mt-5">
<h3>User: useState</h3>
<button className="btn btn-outline-primary" onClick={login}>Login</button>

{user && (
<pre>{JSON.stringify(user, null, 2)}</pre>
)}
</div>
)
}
What’s Happening Behind the Scenes?


When you write useState(), TypeScript infers the type from the initial value. If no value is passed, the type is undefined.

But when you provide a generic, like useState<User | null>(null), TypeScript now expects an object with uid and name fields, or null.

This enables:

• Autocompletion
• Type safety
• Better debugging

Why It Matters


React + TypeScript is powerful, but only when used correctly:

Without Typing	With Typing (User - null)
Error-prone	Type-safe and predictable
No autocompletion	Full IntelliSense for user.uid
Can pass anything to setUser	Only valid User objects or null

Best Practices for useState with Objects

Tip	Description
Always type your state	Especially when managing complex structures
Use - null for optional states	Makes checks like if (user) safe
Prefer interfaces for clarity	Define interface User to keep types reusable
Validate before rendering	Use conditional rendering with optional values

Conclusion


This TypeScript error isn’t just noise—it’s a clue to improve your code's safety and structure. By defining a proper type for your state, you unlock the full power of React + TypeScript.

Don’t let the compiler scare you—embrace types, and make your components clean, robust, and production-ready.

Tags: react typescript useState frontend state-management errors

Пожалуйста Авторизируйтесь или Зарегистрируйтесь для просмотра скрытого текста.