Parent → child data flow in React is props — that's the fundamental, one-directional mechanism.

Props — the basic mechanism

The parent passes data as JSX attributes; the child receives a single props object:

function Parent() {
  const user = { name: "Ada" };
  return <Child name="Ada" age={30} user={user} onSave={handleSave} />;
}

function Child({ name, age, user, onSave }) {  // destructure props
  return <p>{name} is {age}</p>;
}

You can pass anything — strings, numbers, objects, arrays, functions, even JSX/components.

Key properties of props

• One-way / unidirectional — data flows down, parent to child. This is deliberate: predictable data flow.
• Read-only — the child must never mutate props. If it needs to change something, it asks the parent (see below) or keeps its own state.
• Re-render on change — when the parent passes new prop values, the child re-renders.

Child → parent: pass a callback

Since props only flow down, to send data up you pass a function as a prop; the child calls it:

<Child onSelect={(item) => setSelected(item)} />
// child: <button onClick={() => onSelect(thing)}>

The children prop

children is a special prop — whatever you nest between a component's tags:

<Card><p>content</p></Card>   // Card receives <p> as props.children

Avoiding prop-drilling

Passing a prop through many intermediate components that don't use it ("prop-drilling") is a smell. For widely-needed data (theme, current user), use Context — or a state library — so deep descendants read it directly without every layer threading it through.

The framing

"Props — the parent passes data as JSX attributes, the child receives a props object. It's one-way and read-only: data flows down, and the child must never mutate props. You can pass anything, including functions — which is exactly how you send data back up: a callback prop the child invokes. children is the special prop for nested content. And when a prop has to travel through many layers that don't use it, that's prop-drilling — the signal to reach for Context."