print-http-headers.js

<script>
function parseHttpHeaders(httpHeaders) {
    return httpHeaders.split("\n")
     .map(x=>x.split(/: */,2))
     .filter(x=>x[0])
     .reduce((ac, x)=>{ac[x[0]] = x[1];return ac;}, {});
}

var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = parseHttpHeaders(req.getAllResponseHeaders());
// Now we can do:  headers["content-type"]

console.log('HTTP Headers Debug:');
console.log('x-forwarded-user: '     + headers["x-forwarded-user"]);
console.log('x-forwarded-email: '    + headers["x-forwarded-email"]);
console.log('x-auth-request-email: ' + headers["x-auth-request-email"]);
</script>