html

 <a id="btnDownload" class="hide"></a>

JavaScript

var btnDownload = document.getElementById("btnDownload");

var MIME = {
    "application/x-zip-compressed": "zip",
    "application/javascript": "js",
    "text/css": "css",
    "text/plain": "txt",
    "text/html": "html",
    "text/xml": "xml",
    "image/jpeg": "jpeg",
    "image/png": "png",
    "image/gif": "gif",
    "image/svg+xml": "svg"
};

//文件名默认当前时间戳
function downBaseImg(base, name = '下载') {
    var fname = name + "." + MIME[getContentType(base)];
    var blob = getBlob(base);

    if (navigator.msSaveBlob) {
        navigator.msSaveBlob(blob, fname);
    }
    else {
        btnDownload.download = fname;
        btnDownload.href = URL.createObjectURL(blob);
        btnDownload.click();
    }
}

/**
 * 获取Blob
 * @param {stirng} base64
 */
function getBlob(base64) {
    return b64toBlob(getData(base64), getContentType(base64));
}

/**
 * 获取文件类型
 * @param {string} base64
 */
function getContentType(base64) {
    return /data:([^;]*);/i.exec(base64)[1];
}

/**
 * 获取base64中的数据
 * @param {string} base64
 */
function getData(base64) {
    return base64.substr(base64.indexOf("base64,") + 7, base64.length);
}

/**
 * base64转Blob
 * @param {string} b64Data
 * @param {string} contentType
 * @param {number} sliceSize
 */
function b64toBlob(b64Data, contentType, sliceSize) {
    contentType = contentType || '';
    sliceSize = sliceSize || 512;

    var byteCharacters = atob(b64Data);
    var byteArrays = [];

    for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
        var slice = byteCharacters.slice(offset, offset + sliceSize);

        var byteNumbers = new Array(slice.length);
        for (var i = 0; i < slice.length; i++) {
            byteNumbers[i] = slice.charCodeAt(i);
        }

        var byteArray = new Uint8Array(byteNumbers);

        byteArrays.push(byteArray);
    }

    var blob = new Blob(byteArrays, { type: contentType });
    return blob;
}

test

downBaseImg(base64, filename);