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const canvas = document.createElement('canvas')
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// do something...
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canvas.toBlob(function(blob) {
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  var a = document.createElement("a");
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  document.body.appendChild(a);
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  a.download = "img" + ".jpg";
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  a.href = window.URL.createObjectURL(blob);
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4 collapsed lines
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  a.click();
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  document.body.removeChild("a");
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  window.URL.revokeObjectURL(blob)
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})