A key advantage of React is its unidirectional data flow. This makes the flow of data predictable, and helps avoid unexpected side effects from data changing unexpectedly.

But what exactly does “unidirectional data flow” mean in React? Let’s break it down:

The Data Flows Down

In React, parent components pass data to children via props:

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

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

// Child
function Child({value}) {
  return <h1>{value}</h1>;
}

The parent’s value state is passed down into the Child via a prop. This is the “data down” part.

Events Flow Up

When some data needs to change, events fire and bubble up:

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// Child
function Child({value, onUpdate}) {
  return (
    <button onClick={() => onUpdate('World')}>
      Update Value
    </button>
  );
}

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

  const handleUpdate = (newValue) => {
    setValue(newValue);
  }

  return <Child value={value} onUpdate={handleUpdate} />;
}

The onUpdate callback propagates the event up to the parent. This is the “events up” part.

Benefits of Unidirectional Flow

This pattern provides several benefits:

  • Predictable - Only one way data can be modified
  • Modular - Each component only worries about its own state
  • Easy to reason about - Avoid cascading updates across multiple components

Unidirectional flow enforces good React architecture.

Bidirectional Flow Dangers

Other frameworks use two-way binding. This leads to cascading updates that are hard to trace:

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A -> B -> C

B updates 
C updates
A updates from C
B updates again

React’s top-down flow keeps data changing in one place only.

Summary

  • React uses unidirectional data flow
  • Parent components pass data down via props
  • Child components propagate events up
  • This flow prevents cascading updates across components

Learning to structure apps to follow unidirectional data flow takes practice, but leads to more maintainable code.