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

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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