In this post, i'll explain how to pass a state between components in Reactjs. We'll build a small 'How many books did you read?' app, in this application, we have two main components one big called 'Library' and another small 'Book', we have 3 books in the library state and each book has its own state. check any book to count as read. try the app here

Let's begin with code:

In our Library component we have this state

this.state = {
	reads: 0,
	books: [
		{
			name: 'Zero to one',
			isbn: '9780804139298',
			author: 'Peter Thiel',
			cover: 'https://images.gr-assets.com/books/1414347376l/18050143.jpg',
			status: false
		},
		{
			name: "The Manager's Path",
			isbn: '9781491973899',
			author: 'Camille Fournier',
			cover: 'https://images.gr-assets.com/books/1484107737l/33369254.jpg',
			status: false
		},
		{
			name: 'Calculus, Better Explained',
			isbn: '9781470070700',
			author: 'Kalid Azad',
			cover: 'https://images.gr-assets.com/books/1448590460l/27993945.jpg',
			status: false
		}
	]
};

and want to create this.state.books.length - as a number - Book components each have props from the books array of the Library component's state. We have to deal with the two components.

First with the parent, we have to create the Book component this.state.books.length - as a number - times, and pass our diffrent values to them Like this:

Full Code here

{
	this.state.books.map((_book, _id) => {
		return (
			<Book
				handleCounter={this.handleCounter}
				key={_id}
				id={_book.isbn}
				name={_book.name}
				isbn={_book.isbn}
				author={_book.author}
				cover={_book.cover}
			/>
		);
	});
}

Note ignore handleCounter for now.

Second with the child Book, we have the values from parent, let's use them:

Full code here

...
render() {
	return (
		<Card>
			<Image
				src={this.props.cover}
    			alt="Book cover"
...

Until now we created 3 Book components from the parent component Library, and set their values from the parent state. Easy! Right? Great, let's begin the second part

In this section, we want to handle the number of books you read by checking on each book checkbox.

In our Book we have this state

this.state = {
	status: false,
	id: this.props.id
};

Note don't forget to pass props to component's constructor.

status means if you read this book or not, and its default value is false, id is the id of this book and i set it by book id like we learned in the previous section.

we need to handle change of this status then update the books array in the parent state.

In our Book component, we'll add a checkbox that recieve the change of the book status and pass this.handleChange to its onChange event like this:

<input type="checkbox" name="example" onChange={this.handleChange} />

you need to bind the function first, then update the Book state with the new status, after updating the child state we'll update the state of the parent Library like this:

handleChange() {
		this.setState({status: !this.state.status}, this.updateLibraryCount);
    }

updateLibraryCount() {
		this.props.handleCounter(this.state);
	}

In updateLibraryCount we used handleCounter function of the Library as a prop, then passed the Book state to it, Now the Library sees the Book state, Great! let's use it.

handleCounter(_State) {
        //Get the index of this book by searching by its unique isbn
        const ObjNum = this.state.books.findIndex(
			_book => _book.isbn === _State.id
        );

        //then update its value in the Library component
		this.setState(
			{
				books: update(
					ObjNum,
					{...this.state.books[ObjNum], status: _State.status},
					this.state.books
				)
			},
			() => {
                //this is a callback to handle the new change of the book status and increment the reads
				const _read = this.state.books.filter(_book => _book.status === true)
					.length;
				this.setState({reads: _read});
			}
		);
	}

I hope you're understand how to pass the state from the parent to child and vice versa, Here's the full code, and this is the original post on my blog. If you have questions, feel free to ask in comments or email.