fetch.conf

error_log /dev/stdout info;
daemon off;
master_process off;
#worker_processes 1;

events { }

http {
    js_import main from fetch.js;

    resolver 1.1.1.1;

    server {
        listen       8000;

        location / {
            js_content main.fetch;
        }
    }

    server {
        listen       8001;

        location / {
            js_content main.reply;
        }
    }
}

fetch.js

async function fetch(r) {
    let reply = await ngx.fetch("http://127.0.0.1:8001/",
                                { headers: { 'x-ms': "a", 'foo': 'xxx', 'Content-Length': "0" } })

    let body = await reply.text();

    r.return(200, body);
}

function reply(r) {
    let body = [njs.version, '\n'];
    r.rawHeadersIn.forEach((header) => {
        body.push(`${header[0]}: ${header[1]}\n`);
    });

    r.return(200, body.join(''));
}

export default { fetch, reply };