map.js

var ByteArray = require("binary").ByteArray;
var Stream = require("io").Stream;

exports.app = function(env) {
    var stream = new Stream(new java.io.ByteArrayOutputStream());
    // map defined elsewhere
    map.render({
        imageType: "png",
        output: stream
    });
    var body = new ByteArray(stream.unwrap().toByteArray());
    return {
        status: 200,
        headers: {"Content-Type": "image/png"},
        body: [body]
    };    
};