[Solved]-React.js function not being called on component event-Reactjs

are you getting error like ,'EVENTNAME' is not defined no-undef if that's the case, add declaration in front of function definition.

 let EVENTNAME = e => {
          console.log('called', e);
        }

Problem solved define your images as

import React from "react";
import ImageGallery from "react-image-gallery";
import "./styles.css";

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      slidesLoaded: 0
    };
  }

  images = [
    {
      original: "https://picsum.photos/id/1018/1000/600/",
      thumbnail: "https://picsum.photos/id/1018/250/150/"
    },
    {
      original: "https://picsum.photos/id/1015/1000/600/",
      thumbnail: "https://picsum.photos/id/1015/250/150/"
    },
    {
      original: "https://picsum.photos/id/1019/1000/600/",
      thumbnail: "https://picsum.photos/id/1019/250/150/"
    }
  ];

  checkAndShowSlider = e => {
    console.log("called", e);
    let count = this.images.length;
    let nowLoaded = this.state.slidesLoaded + 1;
    if (nowLoaded === count) {
      document.querySelector(".images.section").css("display", "block");
    }
    this.setState(() => {
      return {
        slidesLoaded: nowLoaded
      };
    });
  };

  render() {
    return (
      <div className="App">
        <ImageGallery
          onImageLoad={e => this.checkAndShowSlider(e)}
          items={this.images}
          showPlayButton={false}
          thumbnailPosition="left"
          useBrowserFullscreen={false}
        />
      </div>
    );
  }
}

export default App;