title: React “Uncaught TypeError: Cannot read property of undefined”
body: <p>I have a icon modal component which display material google icon 
  The idea is to send back the clicked icon (letter) from Modal.js to app.js by a callback function someFn which will change state of letter 
  but I get Uncaught TypeError: Cannot read property 'someFn' of undefined everytime I click on an icon.
  I don't understand what I'm doing wrong</p>
  
  <p>Modal.js</p>
  
  <pre><code>import React from 'react';
  import PropTypes from 'prop-types';
   const  NumberList = (props) =&gt; {
      const letters = ['3d_rotation e84d','ac_unit eb3b','access_alarm e190','access_alarms e191','access_time e192','accessibility e84e','accessible e914','account_balance e84f','account_balance_wallet e850','account_box e851','account_circle e853','adb e60e','add e145','add_a_photo e439','add_alarm e193','add_alert e003','add_box e146','add_circle e147','add_circle_outline e148','add_location e567','add_shopping_cart e854','add_to_photos e39d','add_to_queue e05c','adjust e39e','airline_seat_flat e630','airline_seat_flat_angled e631','airline_seat_individual_suite e632','airline_seat_legroom_extra e633','airline_seat_legroom_normal e634','airline_seat_legroom_reduced e635','airline_seat_recline_extra e636','airline_seat_recline_normal e637','airplanemode_active e195','airplanemode_inactive e194','airplay e055','airport_shuttle eb3c','alarm e855','alarm_add e856','alarm_off e857','alarm_on e858','album e019','all_inclusive eb3d','all_out e90b','android e859','announcement e85a','apps e5c3','archive e149','arrow_back e5c4','arrow_downward e5db','arrow_drop_down e5c5','arrow_drop_down_circle e5c6','arrow_drop_up e5c7','arrow_forward e5c8','arrow_upward e5d8','art_track e060','aspect_ratio e85b','assessment e85c','assignment e85d','assignment_ind e85e','assignment_late e85f','assignment_return e860','assignment_returned e861','assignment_turned_in e862','assistant e39f','assistant_photo e3a0','attach_file e226','attach_money e227','attachment e2bc','audiotrack e3a1','autorenew e863','av_timer e01b','backspace e14a','backup e864','battery_alert e19c','battery_charging_full e1a3','battery_full e1a4','battery_std e1a5','battery_unknown e1a6','beach_access eb3e','beenhere e52d','block e14b','bluetooth e1a7','bluetooth_audio e60f','bluetooth_connected e1a8','bluetooth_disabled e1a9','bluetooth_searching e1aa','blur_circular e3a2','blur_linear e3a3','blur_off e3a4','blur_on e3a5','book e865','bookmark e866','bookmark_border e867','border_all e228','border_bottom e229','border_clear e22a','border_color e22b','border_horizontal e22c','border_inner e22d','border_left e22e','border_outer e22f','border_right e230','border_style e231','border_top e232','border_vertical e233','branding_watermark e06b','brightness_1 e3a6','brightness_2 e3a7','brightness_3 e3a8','brightness_4 e3a9','brightness_5 e3aa','brightness_6 e3ab','brightness_7 e3ac','brightness_auto e1ab','brightness_high e1ac','brightness_low e1ad','brightness_medium e1ae','broken_image e3ad','brush e3ae','bubble_chart e6dd','bug_report e868','build e869','burst_mode e43c','business e0af','business_center eb3f','cached e86a','cake e7e9','call e0b0','call_end e0b1','call_made e0b2','call_merge e0b3','call_missed e0b4','call_missed_outgoing e0e4','call_received e0b5','call_split e0b6','call_to_action e06c','camera e3af','camera_alt e3b0','camera_enhance e8fc','camera_front e3b1','camera_rear e3b2','camera_roll e3b3','cancel e5c9','card_giftcard e8f6','card_membership e8f7','card_travel e8f8','casino eb40','cast e307','cast_connected e308','center_focus_strong e3b4','center_focus_weak e3b5','change_history e86b','chat e0b7','chat_bubble e0ca','chat_bubble_outline e0cb','check e5ca','check_box e834','check_box_outline_blank e835','check_circle e86c','chevron_left e5cb','chevron_right e5cc','child_care eb41','child_friendly eb42','chrome_reader_mode e86d','class e86e','clear e14c','clear_all e0b8','close e5cd','closed_caption e01c','cloud e2bd','cloud_circle e2be','cloud_done e2bf','cloud_download e2c0','cloud_off e2c1','cloud_queue e2c2','cloud_upload e2c3','code e86f','collections e3b6','collections_bookmark e431','color_lens e3b7','colorize e3b8','comment e0b9','compare e3b9','compare_arrows e915','computer e30a','confirmation_number e638','contact_mail e0d0','contact_phone e0cf','contacts e0ba','content_copy e14d','content_cut e14e','content_paste e14f','control_point e3ba','control_point_duplicate e3bb','copyright e90c','create e150','create_new_folder 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        const listItems = letters.map((letter,index) =&gt;
         &lt;a key={index} onClick={props.someFn}&gt;
          &lt;i className ="material-icons" &gt;{letter}&lt;/i&gt;
         &lt;/a&gt;
        );
        return &lt;span&gt;{listItems}&lt;/span&gt;;
  
  };
  
  class Modal extends React.Component {
    constructor(props) {
       super(props);
  
       this.someFn = this.someFn.bind(this); 
  
     }
  
        someFn = (clickEvent) =&gt; {
  
  
          this.props.callbackFromParent(letter);
      }
  
  
    render() {
      // Render nothing if the "show" prop is false
      if(!this.props.show) {
        return null;
      }
  
      // The gray background
      const backdropStyle = {
        position: 'fixed',
        top: 0,
        bottom: 0,
        left: 0,
        right: 0,
        backgroundColor: 'rgba(0,0,0,0.3)',
        padding: 50
      };
  
  
  
      // The modal "window"
      const modalStyle = {
        backgroundColor: '#fff',
        borderRadius: 5,
        maxWidth: 500,
        minHeight: 300,
        margin: '0 auto',
        padding: 30
      };
  
      return (
        &lt;div className="backdrop" style={backdropStyle}&gt;
          &lt;div className="modal" style={modalStyle}&gt;
            &lt;NumberList someFn={this.someFn} /&gt;
            &lt;div className="footer"&gt;
              &lt;button onClick={this.props.onClose}&gt;
                Close
              &lt;/button&gt;
            &lt;/div&gt;
          &lt;/div&gt;
        &lt;/div&gt;
      );
    }
  }
  
  Modal.propTypes = {
    onClose: PropTypes.func.isRequired,
    show: PropTypes.bool,
    callbackFromParent: PropTypes.func.isRequired,
  
  };
  
  export default Modal;
  </code></pre>