html2canvas/src/proxy.js

function Proxy(src, proxyUrl, document) {
    var callback = createCallback(supportsCORS);
    var url = createProxyUrl(proxyUrl, src, callback);

    return supportsCORS ? XHR(url) : (jsonp(document, url, callback).then(function(response) {
        return decode64(response.content);
    }));
}
var proxyCount = 0;

var supportsCORS = ('withCredentials' in new XMLHttpRequest());
var supportsCORSImage = ('crossOrigin' in new Image());

function ProxyURL(src, proxyUrl, document) {
    var callback = createCallback(supportsCORSImage);
    var url = createProxyUrl(proxyUrl, src, callback);
    return (supportsCORSImage ? Promise.resolve(url) : jsonp(document, url, callback).then(function(response) {
        return "data:" + response.type + ";base64," + response.content;
    }));
}

function jsonp(document, url, callback) {
    return new Promise(function(resolve, reject) {
        var s = document.createElement("script");
        var cleanup = function() {
            delete window.html2canvas.proxy[callback];
            document.body.removeChild(s);
        };
        window.html2canvas.proxy[callback] = function(response) {
            cleanup();
            resolve(response);
        };
        s.src = url;
        s.onerror = function(e) {
            cleanup();
            reject(e);
        };
        document.body.appendChild(s);
    });
}

function createCallback(useCORS) {
    return !useCORS ? "html2canvas_" + Date.now() + "_" + (++proxyCount) + "_" + Math.round(Math.random() * 100000) : "";
}

function createProxyUrl(proxyUrl, src, callback) {
    return proxyUrl + "?url=" + encodeURIComponent(src) + (callback.length ? "&callback=html2canvas.proxy." + callback : "");
}