JavaScript function to save the contents of canvas element to a file.

saveCanvas.js

window.URL = window.URL || window.webkitURL || window.mozURL || window.msURL;
window.saveAs = window.saveAs || window.webkitSaveAs || window.mozSaveAs || window.msSaveAs;

function saveCanvas (canvas, filename, fileformat) {
	if (navigator.msSaveBlob || window.URL || window.saveAs) {
		if (canvas.toBlob) {
			canvas.toBlob(function (blob) {
				saveBlob(blob, filename);
			}, fileformat);
		}
		else {
			saveBlob(dataURLToBlob(canvas.toDataURL(fileformat)), filename);
		}
	}
	else {
		saveUrl(canvas.toDataURL(fileformat), filename);
	}
}

function dataURLToBlob (dataURL) {
	var index = dataURL.indexOf(',');
	var meta = dataURL.substring(0, index);
	var data = dataURL.substring(index + 1);
	var contentType = meta.substring(meta.indexOf(':') + 1);

	if (/;base64$/.test(contentType)) {
		contentType = contentType.substring(0, contentType.length - 7);
		var strdata = atob(data);

		data = new Uint8Array(strdata.length);

		for (var i = 0; i < strdata.length; ++ i) {
			data[i] = strdata.charCodeAt(i);
		}
	}
	else {
		data = decodeURIComponent(data);
	}

	return new Blob([data], {type: contentType});
}

function saveBlob (blob, filename) {
	if (navigator.msSaveBlob) {
		navigator.msSaveBlob(blob, filename);
	}
	else if (window.saveAs) {
		window.saveAs(blob, filename);
	}
	else {
		var url = window.URL.createObjectURL(blob);

		saveUrl(url, filename);

		setTimeout(function () {
			window.URL.revokeObjectURL(url);
		}, 250);
	}
}

function saveUrl (url, filename) {
	var link = document.createElement("a");
	if ('download' in link) {
		link.download = filename;
		link.href = url;
		link.style.position = 'absolute';
		link.style.left = '0';
		link.style.top = '0';

		// some browsers need it to be in the document
		document.body.appendChild(link);
		link.click();
		
		setTimeout(function () {
			document.body.removeChild(link);
		}, 250);
	}
	else {
		// async callback -> window.open() will fail
		window.location = url;
	}
}