Why is my Canvas drawImage() failing to render images, and how can I resolve onload and CORS issues?

CanvasContext2D drawImage() conundrum: Tackling onload and CORS roadblocks

Problem Presentation:

Embarking on a canvas painting escapade, you find yourself faced with a baffling conundrum. Images rendered via drawImage() elude your grasp, leaving behind empty data URLs. Moreover, attempts to display the canvas yield no visible results, and the console remains eerily silent.

Unveiling the Perplexity:

The crux of the issue lies in the premature nature of your canvas artistry. Before drawImage() can wield its magic, the image must first complete its loading process. To appease this onLoad necessity, embrace the following strategy:

// Create the image
var img = new Image();

// Define the onload function
img.onload = function() {
  // Canvas setup
  var canvas = document.createElement("canvas");
  canvas.width = img.width;
  canvas.height = img.height;
  var context = canvas.getContext("2d");

  // Draw the image
  context.drawImage(img, 0, 0);

  // Obtain the data URL
  var dataURL = canvas.toDataURL();

  // Perform desired actions with dataURL
  doSomething(dataURL);
};

// Set the image source
img.src = "http://somerandomWebsite/picture.png";

CORS Conundrum and Mitigation:

To ensure seamless toDataURL() and getImageData() execution, adhering to cross-origin resource sharing (CORS) is imperative. Prevent canvas 'tainting' by ensuring one of the following scenarios:

• Same-server image origin.
• External server with permissive CORS headers and img.crossOrigin set to "use-credentials".
• Anonymous CORS server with img.crossOrigin set to "anonymous".

Important Notes:

• CORS headers originate from the server, and the cross-origin attribute merely requests CORS usage.
• IE and Safari lack support for the cross-origin attribute.

Edge Case: Discrepancies in Image Sources:

If your images stem from a mix of same-server and CORS-compliant sources, consider utilizing the onerror event handler. Assign cross-origin as 'anonymous' to non-CORS servers and listen for errors.

function corsError() {
  this.crossOrigin = "";
  this.src = "";
  this.removeEventListener("error", corsError, false);
}
img.addEventListener("error", corsError, false);

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