SOURCE

function exportCanvasAsPNG(id, fileName) {
 
    var canvasElement = document.getElementById(id);
 
    var MIME_TYPE = "image/png";
 
    var imgURL = canvasElement.toDataURL(MIME_TYPE);
 
    var dlLink = document.createElement('a');
    dlLink.download = fileName;
    dlLink.href = imgURL;
    dlLink.dataset.downloadurl = [MIME_TYPE, dlLink.download, dlLink.href].join(':');
 
    document.body.appendChild(dlLink);
    dlLink.click();
    document.body.removeChild(dlLink);
}

<button onclick="exportCanvasAsPNG('pieChart','nianling')">保存</button>