The Problem with Mutating Props in React

A fundamental rule in React is that props should be immutable – child components should never modify their props directly. Here’s why and how to handle this properly:

Why Mutating Props is Bad

// ❌ Wrong - directly mutating props
this.props.someValue = 'new value';

1. React’s Unidirectional Data Flow: Props should only flow down from parent to child
2. Predictability Issues: Makes data flow harder to track and debug
3. Performance Problems: Bypasses React’s change detection system
4. Potential Bugs: Parent component won’t know about the change

Correct Patterns

1. Lift State Up (Recommended)

Have the parent component own the state and pass down both the value and a handler:

// Parent Component
function Parent() {
  const [value, setValue] = useState('initial');

  return <Child value={value} onChange={setValue} />;
}

// Child Component
function Child({ value, onChange }) {
  return (
    <input 
      value={value}
      onChange={(e) => onChange(e.target.value)}
    />
  );
}

2. Use Callback Functions

When you need to modify data that comes from props:

// Parent Component
class Parent extends React.Component {
  state = { items: ['a', 'b', 'c'] };

  handleItemUpdate = (index, newValue) => {
    this.setState(prev => ({
      items: prev.items.map((item, i) => 
        i === index ? newValue : item
      )
    }));
  };

  render() {
    return (
      <Child 
        items={this.state.items} 
        onUpdate={this.handleItemUpdate} 
      />
    );
  }
}

// Child Component
function Child({ items, onUpdate }) {
  const handleClick = (index) => {
    // ✅ Correct - Notifies parent through callback
    onUpdate(index, 'updated');
  };

  return (
    <div>
      {items.map((item, i) => (
        <button key={i} onClick={() => handleClick(i)}>
          {item}
        </button>
      ))}
    </div>
  );
}

3. Clone Props When Needed (Rare Cases)

In some edge cases where you need to work with prop values locally:

function Child({ initialValue }) {
  const [localValue, setLocalValue] = useState(initialValue);

  // Now you can modify localValue without affecting props
  return (
    <input
      value={localValue}
      onChange={(e) => setLocalValue(e.target.value)}
    />
  );
}

Special Case: Object/Array Props

Be extra careful with object/array props – even seemingly safe operations might mutate:

// ❌ Dangerous - might mutate props
const newArray = props.items;
newArray.push('new item');

// ✅ Safe - creates new copy
const newArray = [...props.items, 'new item'];

Why React Enforces This

1. Single Source of Truth: Data should be owned by one component
2. Predictable Updates: Changes should always flow top-down
3. Performance Optimization: Helps React determine when to re-render
4. Debugging Simplicity: Makes data flow easier to follow

Modern Alternative: State Management Libraries

For complex cases, consider using state management solutions that provide controlled mutation patterns:

// Using Redux
const mapDispatchToProps = (dispatch) => ({
  updateItem: (payload) => dispatch(updateAction(payload))
});

// Using Context API
const { updateValue } = useContext(MyContext);

Remember: In React, props are read-only. Any data that needs to be changed should be managed by the component that owns the state, with changes communicated upward through callbacks or state management systems.