xhr_spy.js

const functionsToOverride = ['overrideMimeType', 'send'];
functionsToOverride.forEach(function(fnName) {
    window.XMLHttpRequest.prototype[fnName] = new Proxy(window.XMLHttpRequest.prototype[fnName], {
        apply: (target, thisArg, argumentsList) => {
            console.log(fnName + ' arguments', argumentsList);
            thisArg[fnName + 'Args'] = [...argumentsList];
            const retVal = target.apply(thisArg, argumentsList);
            console.log(fnName + ' retVal', retVal);
            return retVal;
        }
    });
});

window.XMLHttpRequest.prototype.open = new Proxy(window.XMLHttpRequest.prototype.open, {
    apply: (target, thisArg, argumentsList) => {
        console.log('open arguments', argumentsList);
        thisArg.openArgs = [...argumentsList];

        const listenEvents = ['onreadystatechange', 'abort', 'error', 'load', 'loadstart', 'loadend', 'progress', 'timeout'];
        listenEvents.forEach(function(eName) {
            this.addEventListener(eName, function() {
                console.log('event ' + eName + ' arguments', arguments);
            });
        }, thisArg);

        const retVal = target.apply(thisArg, argumentsList);
        console.log('open retVal', retVal);
        return retVal;
    }
});

window.XMLHttpRequest.prototype.setRequestHeader = new Proxy(window.XMLHttpRequest.prototype.setRequestHeader, {
    apply: (target, thisArg, argumentsList) => {
        console.log('setRequestHeader arguments', argumentsList);
        if (!this.setRequestHeaderArgs) {
            thisArg.setRequestHeaderArgs = new Array();
        }
        thisArg.setRequestHeaderArgs.push([...argumentsList]);
        var retVal = target.apply(thisArg, argumentsList);
        console.log('setRequestHeader retVal', retVal);
        return retVal;
    }
});

// Standard properties containing info about the request
// XMLHttpRequest.readyState
// XMLHttpRequest.response
// XMLHttpRequest.responseText
// XMLHttpRequest.responseType
// XMLHttpRequest.responseURL
// XMLHttpRequest.responseXML
// XMLHttpRequest.status
// XMLHttpRequest.statusText
// XMLHttpRequest.timeout
// XMLHttpRequest.withCredentials