title: can't setState in a function inside a arrow function on React
body: <p>I'm trying to use <code>This</code> inside a function on a arrow function. How can I use <code>setState</code> inside my function?</p>
  
  <pre><code>showDeleteConfirm = commentId =&gt; {
      const { deleteComment } = this.props;
      const { comments } = this.state;
      // this exist
      confirm({
        title: 'Are you sure delete this?',
        okText: 'Yes',
        okType: 'danger',
        cancelText: 'No',
        onOk() {
          // this is undefined
          deleteComment(commentId);
          const targetPosition = _.findIndex(comments, item =&gt; {
            return item._id === commentId;
          });
          if (targetPosition !== -1) {
            // this is undefined
            console.log(this.state.comments);
            this.setState(prevState =&gt; ({
              comments: [
                ...prevState.comments.slice(0, targetPosition),
                ...prevState.comments.slice(targetPosition + 1)
              ]
            }));
          }
        },
        onCancel() {
          console.log('Cancel');
        }
      });
    };
  </code></pre>