REACT: How do you set a child component's state to its parent's state?

You can pass the part of the parent state that you need as a prop to the child. Then each time the parent state will change, the child will rerender with the correct value.

If you need to change the state from within the child, it depends on the behaviour you want.

You can make the child change the parent state by passing a callback function as a prop (you can pass the function used to change the state in the parent as a prop to the child)

Or you can make the child local state beeing reset to the parent state when it changes by listening to changes on the prop with a useEffect or ComponentDidUpdate.

useEffect(() => { setState(props.partOfparentState)},[props.partOfparentState])

or

ComponentDidUpdate(prevProps) {
    if(previousProps.partOfParentState != props.partOfParentState) {
        partOfParentStatethis.setState({state:props.parpartOfParentStatetOfParentState})
    }
}

You could also want other behaviour which could be achieved with a more complex useEffect.

State And Props

React DOM compares the element and its children to the previous one, and only applies the DOM updates necessary to bring the DOM to the desired state.

the data flow in react is “unidirectional” or “top-down”, Any state is always owned by some specific component, and any data or UI derived from that state can only affect components “below” them in the tree.

If you imagine a component tree as a waterfall of props, each component’s state is like an additional water source that joins it at an arbitrary point but also flows down.

This is why the state is often called local or encapsulated. It is not accessible to any component other than the one that owns and sets it. @reactjs

Therefore, You Should pass the state to the child component, also known as prop.

Edit:

the hierarchy will be like this: App(state - date) => Child1(prop date, passing the same prop to the next child) => Child2(getting date from his parent - Child1)

Example (based on Class Components):

<App>:

The root of the app.

<ClockList date={this.state.date} />

<ClockList>:

notice we are using props

<Clock date={this.props.date} />

<Clock>:

import React from "react";

const Clock = (props) => {
  // Object Destructuring can also be done in the function itself.
  const { date } = props;
  return (
    <div className="container">
      <p>{date}</p>
    </div>
  );
};

export default Clock;