How to Fix Mutation Bugs in Your React Complex State Logic

Have you ever encountered mysterious bugs in your React application when working with complex state management? You know, those frustrating moments when your state updates don't quite work as expected, leaving you scratching your head?

State mutation bugs are like silent assassins in React development - they can create unpredictable behavior that's incredibly difficult to debug. Many developers, even experienced ones, fall into this trap without realizing it.

In this comprehensive guide, we'll dive deep into the world of React state mutations. You'll learn why these bugs occur, how to identify them, and most importantly, the proper techniques to prevent them. Whether you're working with arrays, objects, or nested data structures, you'll discover the best practices for handling complex state updates.

Complex state

I have already written a bunch of blog post explaining how we can store numbers and strings in React state. But in an enterprise world, our state will be in a more complex shape, like an object or an array.

Take the case, we create a colors state variable. It holds an array of strings:

const [colors, setColors] = React.useState(['#FFD500', '#FF0040']);

React doesn't care what type our state is. We can store numbers or strings or arrays or objects. We can even store functions in React state if it pleases us!

But there's a catch: React state changes have to be immutable. When we call setColors, we need to provide a brand-new array. We should never mutate arrays or objects held in state.

Never mutate React state

So to have a solution for the code above, the order of operations is:

1. Create a new array
2. Modify that new array
3. Set the new array into state

// ✅ Best solution
onChange={event => {
  const nextColors = [...colors];
  nextColors[index] = event.target.value;

  setColors(nextColors);
}}

Hmm, you might be wondering: is it OK to flip the order of the first two steps? What if we modify the array, then clone it? Like this:

// ❌ bad idea
<input
  onChange={event => {
    // Mutate the array:
    colors[index] = event.target.value;

    // Create a new copy, and set it into state:
    setColors([...colors]);
  }}
/>

Seems a bit simpler, right? Make whatever modifications we want, and then copy the array right before we call setColors so that we're providing a new value. And if you try this in the above sandbox, it appears to work?

Here's the problem → By doing it this way, we're modifying the values held in React state. React really doesn't expect us to do this, and there are no guardrails in place to warn us if we try.

While you might get away with it once, making this a habit will likely lead to strange, glitchy behavior. Parts of your page might fail to update properly, or DOM elements could unexpectedly appear in completely different locations than intended.

These bugs are really hard to diagnose and fix 🐞. You'll save yourself a whole lot of trouble if you do your best to avoid mutating React state.