moving-graph-3.js

// ./components.App.js

    // decrement function
    handleIncrement = counter => {
        // copy the `counters`
        const counters = [...this.state.counters];
        // indexOf corresponding eventActions
        const index = counters.indexOf(counter);
        // update the `counters` Object using new `counter`
        counters[index] = {...counter};
        counters[index].value++;
        // setState at last
        this.setState({ counters });
    }

    // increment function
    handleDecrement = counter => {
        // copy the `counters`
        const counters = [...this.state.counters];
        // indexOf corresponding eventActions
        const index = counters.indexOf(counter);
        // update the `counters` Object using new `counter`
        counters[index] = {...counter};
        counters[index].value--;
        // setState at last
        this.setState({ counters });
    }