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Multiple unhandled/uncaught promise errors in console #1502

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mlev opened this issue Apr 12, 2018 · 3 comments
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Multiple unhandled/uncaught promise errors in console #1502

mlev opened this issue Apr 12, 2018 · 3 comments

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@mlev
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mlev commented Apr 12, 2018

Bug reports:

Latest version produces multiple unhandled errors in the console - even though canvas appear to render correctly.

This can be reproduced on the demo site (https://html2canvas.hertzen.com/).

Tested on Chrome and Safari - both with the same result.
screen shot 2018-04-12 at 11 39 14 am
screen shot 2018-04-12 at 11 46 12 am

Specifications:

  • html2canvas version tested with:
  • Browser & version: Chrome 65, Safari 11.1
  • Operating system: Mac High Sierra
@tarjeieo
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I see the same in both Chrome and Internet Explorer, on a Windows 10 PC.

@tarjeieo tarjeieo mentioned this issue Apr 16, 2018
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@earshinov
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earshinov commented Feb 22, 2019 •
edited

We experience this, too, which is very annoying, particularly because we have Sentry enabled and each unhandled promise rejection results in an error notification.

For us, the issue seems to be caused by hidden (zero-sized) frames present on the page. In html2canvas, rendering these frames results in canvas.toDataUrl() returning data:, (see StackOverflow question). When put into ImageElement.src, such data URL causes an onerror event, which html2canvas ties to the reject Promise callback here:

html2canvas/src/Clone.js

Line 216 in 41eb8ab

iframeCanvas.onerror = reject;

I have prepared a demo page. Unfortunately, due to browser's same-origin policy and additional restrictions set up on JSFiddle, you have to store this page somewhere and have it served by a web server (python -m http.server will suffice):

index.html:

<!DOCTYPE html>
<script src="http://unpkg.com/html2canvas@1.0.0-alpha.12/dist/html2canvas.js"></script>
<iframe src="/" style="display: none;" width="0" height="0"></iframe>
<button onclick="run()">Call html2canvas</button>
<script src="script.js"></script>

script.js:

(function() {
  function setup() {
    window.addEventListener('unhandledrejection', event => {
    	alert('Unhandled rejection');
    	console.error('Unhandled Promise rejection', event);
    }, false);

    const _Promise = window.Promise;
    const Promise = function(_cb) {
      if (new.target) {
        return new _Promise(function(resolve, _reject) {
          function reject(value) {
            debugger;
            console.error("Rejecting a Promise with value", value);
            return _reject(value);
          };
          return _cb(resolve, reject);
        });
      }
      return _Promise.apply(this, arguments);
    }
    // copy members
    for (const prop of Object.getOwnPropertyNames(_Promise))
      Object.defineProperty(Promise, prop, Object.getOwnPropertyDescriptor(_Promise, prop));
    window.Promise = Promise;
  }

  function run() {
  	html2canvas(document.body, {
      width: window.innerWidth,
    	height: window.innerHeight,
      windowWidth: window.innerWidth,
      windowHeight: window.innerHeight,
    }).then(canvas => {
    	const img = document.createElement('img');
      img.src = canvas.toDataURL();
      document.body.appendChild(img);
    });
  }

  setup();

  window.run = run;
})();

image

To me, ignoring onerror in case of an empty data URL inside html2canvas seems a feasible solution:

Clone.js:

//iframeCanvas.onerror = reject;
iframeCanvas.onerror = function (event) {
    // Empty iframes may result in empty "data:," URLs, which are invalid from the <img>'s point of view
    // and instead of `onload` cause `onerror` and unhandled rejection warnings
    iframeCanvas.src == 'data:,' ? resolve(canvas) : reject(event);
};

earshinov pushed a commit to earshinov/html2canvas that referenced this issue Feb 24, 2019
Prevent unhandled promise rejections for hidden frames
07091e1 
niklasvh#1502
@earshinov earshinov mentioned this issue Feb 24, 2019
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@niklasvh
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niklasvh commented Apr 7, 2019

Fixed in #1762

@niklasvh niklasvh closed this as completed Apr 7, 2019
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4 participants
@earshinov @niklasvh @tarjeieo @mlev