The 4 Most Common Mistakes When Using useState in React

1. Using a Function to Initialize useState

When initializing state with useState, if the initial state is the result of an expensive function, this function will be called on every render. This can severely impact performance, especially if the function involves complex computations.

Incorrect usage:

const [data, setData] = useState(expensiveFunction());

In this example, expensiveFuncttion is called on every render, leading to unnecessary performance costs.

Correct usage:

const [data, setData] = useState(() => expensiveFunction());

By using a function inside useState, the expensiveFuncttion is only called during the initial render, saving resources on subsequent renders.

2. Misusing the setState Function

When updating state in React, especially when the new state depends on the previous state, it's important to use the functional form of setState. A common mistake is updating state without taking the previous value into account.

Incorrect usage:

setCount(count + 1);

This can cause issues if multiple updates are triggered in quick succession because count might not have the most updated value.

Correct usage:

setCount(prevCount => prevCount + 1);

This approach ensures that the state is updated based on its previous value, making it more reliable.

3. Incorrect Placement of useState

Placing hooks inside loops, conditions, or nested functions can break React's state management mechanism.

Incorrect usage:

if (condition) {
  const [value, setValue] = useState(false);
}

This violates React's rules because useState should not be placed inside conditional statements

Correct usage:

const [value, setValue] = useState(false);

if (condition) {
  // Use the state here
}

By keeping useState outside of conditions or loops, React can properly manage the order in which hooks are called.

4. Mutating State Directly

State in React should be treated as immutable. Directly mutating the state can lead to unexpected bugs and issues, particularly when dealing with objects or arrays.

Incorrect usage:

const [user, setUser]= useState({ name: 'John', age: 35 });
user.name = 'Jane';  // Incorrect mutation
setUser(user);       // Incorrect update

Mutating user directly doesn't trigger a re-render because React doesn't detect the state change.

Correct usage:

setUser(prevUser => ({
  ...prevUser,
  name: 'Jane'
}));

Here, a new object is created using the spread operator, ensuring the immutability of state and triggering a proper re-render.