How to save a png from javascript variable

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I know this question is 2 years old, but hopefully people will see this update.

You can prompt the user to save an image in a base64 string (and also set the filename), without asking the user to do a right click

var download = document.createElement('a');
download.href = dataURI;
download.download = filename;
download.click();

Example:

var download = document.createElement('a');
download.href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==";
download.download = 'reddot.png';
download.click();

In order to trigger a click event using Firefox, you need to do what it is explained in this SO answer. Basically: 

function fireEvent(obj,evt){
  var fireOnThis = obj;
  if(document.createEvent ) {
    var evObj = document.createEvent('MouseEvents');
    evObj.initEvent( evt, true, false );
    fireOnThis.dispatchEvent( evObj );
  } else if( document.createEventObject ) {
    var evObj = document.createEventObject();
    fireOnThis.fireEvent( 'on' + evt, evObj );
  }
}
fireEvent(download, 'click')

As of 20/03/2013, the only browser that fully supports the download attribute is Chrome. Check the compatibility table here

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