Calling setState() with a function but not using previous state

When using React’s setState() with a function argument but not actually utilizing the previous state, you’re potentially introducing unnecessary complexity and minor performance overhead. Here’s what you need to know:

The Anti-Pattern

// ❌ Unnecessary function form
this.setState(() => ({
  count: 42
}));

Why This is Problematic

1. Unnecessary Function Invocation: React has to call your function just to get the new state
2. Reduced Readability: Makes code harder to understand at a glance
3. Potential Confusion: Other developers might expect you to use prevState
4. Micro-Performance Impact: Extra function call has a tiny cost

Correct Approaches

1. Simple Object Form (Recommended)

// ✅ Preferred when not using prevState
this.setState({ count: 42 });

2. When You Actually Need Functional Updates

Only use the function form when you need previous state:

// ✅ Correct usage of functional form
this.setState(prevState => ({
  count: prevState.count + 1
}));

Function Component Equivalent

The same principles apply to useState hooks:

const [count, setCount] = useState(0);

// ❌ Unnecessary function
setCount(() => 42);

// ✅ Better
setCount(42);

// ✅ Only use function when needed
setCount(prev => prev + 1);

When Function Form is Acceptable Without prevState

There are rare cases where you might intentionally use the function form without prevState:

1. Ensuring Fresh State in async scenarios (though there are better patterns)
2. Consistent Coding Style if your team prefers it
3. Future-Proofing if you might need prevState later

Performance Considerations

While the performance difference is negligible in most cases, the object form is technically:

1. Faster to Execute: No extra function call
2. Easier to Optimize: For React’s internals
3. More Predictable: For React’s batching mechanism

Best Practice Recommendation

Always use the simplest form that meets your needs:

1. Object form for simple updates
2. Function form when you need previous state or sequential updates
3. Avoid function form when it provides no benefit