Sometimes we need to add a method to a class component and pass it down to its child component.
For example, we may have a GroceryList component that has a child component called GroceryItem.
We define a toggleItem() method in GroceryList that we want to trigger from GroceryItem. How do we do that?

Let's first define the method in the parent component.
The way we add a method to a class component is to simply create an identifier inside the class definition and assign an arrow function to it.
The identifier will be the name of the method we call.

class GroceryList extends React.Component {

  toggleItem = itemId => {
    // perform a toggle action
  }

}

To use this method in the child component, we need to pass it down via props:

<GroceryItem item={item} toggleItem={this.toggleItem} />

Inside the child component we then use this method in an onClick event:

<div onClick={ () => {props.toggleItem(props.item.id)} } />
  Click me
</div>

When we click on the <div> element now we trigger the onClick event that will call toggleItem() in the parent component, passing the item id as an argument.