React downloads pending images of previous render

Question

_{(Update: previously, I thought this problem was caused by React Router, but I have stripped React Router out of the code, and the problem still persists. So I have modified this question thoroughly.)}

Situation:

I have paginated pages which show a list of images per page. (With "page" I mean the complete content that is visible, I don't mean a separate html file/url.) I would like to nagivate through those pages in an efficient manner.

Problem:

If I navigate quickly enough through the pages, not all images will be loaded in the current page before navigating to the next page. I would expect the browser to cancel all pending unfinished image downloads when navigating to the next page. But this doesn't happen, the browser keeps all the unfinished images pending to be downloaded, until they are all downloaded. Then the images of the page to which I navigated, will be downloaded. This causes a big delay and wasted bandwidth.

Question:

Is it possible to cancel the downloading of "pending" images of the previous page?

Demonstration code:

To test this, use the "network" tab in the developer tool of you browser. Also choose "disable cache" and set the throttle (download speed in developer tool) to something slow like "Slow 3G" otherwise the images will be loaded to fast too see the problem. Then navigate through the pages and see that the list of pending images is stacking up, then click the "disable images" button. Then, no image will be visible on the screen, but the browser still has a large number of pending image downloads open, which is wasted bandwith and causes a delay when new images need to be rendered.

You can test the code here: https://codepen.io/Devabc/pen/PowjqwZ

//This code is using imgur images to demonstrate.
class Page extends React.Component {
  state = {
    pageNr: 1,
    imagesEnabled: true
  };

  onLinkClick = event => {
    const number = event.target.dataset.value;
    console.log("Number: " + number);
    this.setState({pageNr: number});
  };

  onButtonClick = event => {
    console.log("toggling images");
    this.setState(prevState => {
      return {imagesEnabled: ! prevState.imagesEnabled};
    });   
  };
 
  render() {
    const links = _.range(0, 5).map(number => {
      return (
        <a href="#" data-value={number} onClick={this.onLinkClick}>
          {number}
        </a>
      );
    });
    
    const pageNr = this.state.pageNr;
    const imgStart = pageNr * 100;
    const imgEnd = imgStart + 100;
    const images = this.state.imagesEnabled 
      ? <ImagesPanel imgStart={imgStart} imgEnd={imgEnd} />
      : null;
    
    return (
      <div>
        <h1>Page {this.state.pageNr}</h1>
        <div>Page links: {links}</div>
        <div><button onClick={this.onButtonClick}>{this.state.imagesEnabled ? "disable images" : "enable images"}</button></div>
        <div>Images:</div>
        <div>{images}</div>
      </div>
    );    
  }
}

function ImagesPanel(props) {  
  const images = _.range(props.imgStart, props.imgEnd).map(number => {
    return (
      <Image imageNr={number} />
    );
  });
  
  return images;
}

function imageUrl(imageNr) {
  const hash = imgurHashes[imageNr];
  return "https://i.imgur.com/" + hash + ".jpg";
}

function Image(props) {
  const url = imageUrl(props.imageNr);  
  return <img class="myImage" src={url} border='1' />
}


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];

ReactDOM.render(
  <Page />,
  document.getElementById('root')
);

The screenshot below shows the list of pending images in the browser, even though no images should be rendered at this moment, because the React component that should render the images is already unmounted/removed.

Answer 1

The problem is not caused by React. I'm not sure who to blame for this problem, it could be the Document Object Model standard, HTML or the browsers implementing it. The problem is that once an image is attached to the DOM (when the browser starts to render a HTML page), the browsers will start to download it, even when the image is later removed from the DOM.

There are however a number of solutions to solve this problem.

Solution 1: src attribute modification to cancel downloads

The src attribute is mandatory in the img element of HTML, but setting it to an empty string (clearing), results in cancellation of the download of the image. (This may not work on all browsers.)

This solution is described here: How to cancel an image from loading and demonstrated here https://jsbin.com/zakeqaseru/1/edit?output and here https://jsfiddle.net/nw34gLgt/ (Use the browser developer tools to monitor whether the image download is cancelled.)

The src can be changed to the empty string using setAttribute(). Setting it to null has the same effect as setting it to the string "null", which would also work, but is less clean. Removing the src attribute doesn't work on most browsers.

Whether this solution works may differ per browser. Older browsers may cause the web page itself to be requested when src is removed, as mentioned here: https://gtmetrix.com/avoid-empty-src-or-href.html This could be a very big performance hit on both the brower and the web server.

In case the browser cannot deal with this, a similar solution would be to use the background-image property on a HTML element like a div instead of an img.

This src-clearing solution is implemented in React-Image: https://github.com/mbrevda/react-image/pull/223

this.i.src = ''

try {
  delete this.i.src
} catch (e) {
  // On Safari in Strict mode this will throw an exception,
  //  - https://github.com/mbrevda/react-image/issues/187
  // We don't need to do anything about it.
}
delete this.i

A solution in React would be something like this:

class Img extends React.Component {
    constructor(props) {
        super(props);

        this.imgRef = React.createRef();
    }

    componentWillUnmount() {
        try {
            this.imgRef.current.src = '';
            delete this.imgRef.current.src;
        } catch (e) {}
    }

    render() {
        return (
            <img {...this.props} ref={this.imgRef} />
        );
    }
}

Then you can use it like this:

<Img src="...some image url...." />

Solution 2: lazy loading src

By setting the src attribute under a programmatic condition, you can make sure that images are only loaded when needed. This solution can also be combined with other solutions to make sure that images that are downloaded, are cancelled when the associated React elements are unmounted.

The data-src (custom data) attribute is often used in this approach, to copy its value to src when an image needs to be rendered.

This solution may still download unmounted images.

Solution 3: Chrome's native lazy-loading

This solution exists since Chrome 76 (July 2019?). Chrome has a loading attribute for images and iframes, which can be set to "lazy" to defer downloading of the resource until it reaches a calculated distance from the viewport.

The loading attribute isn't supported by many other browser.

For more info, see:

• https://web.dev/native-lazy-loading/
• https://github.com/scott-little/lazyload/
• Lazy-loading request for Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1542784

This solution may still download unmounted images.

Additional information sources

• https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video
• https://imagekit.io/blog/lazy-loading-images-complete-guide/