var http = require('http');


const PORT = 25000;
var responses = {};


function handle(request, response) {
    var msize = request.url.substr(1);
    if (!msize) {
        msize = 1024;
    } else {
        msize = parseInt(msize, 10);
    }
    if (!responses[msize]) {
	// for Node.js v4
	responses[msize] = new Buffer(msize).fill("X");
	// for Node.js v6
	// responses[msize] = Buffer.alloc(msize, "X");
    }
    response.end(responses[msize]);
}


var server = http.createServer(handle);

server.listen(PORT, function() {
    console.log('Serving on ::' + PORT.toString());
});

var http = require('http');


const PORT = 25000;
var responses = {};


function handle(request, response) {
    var msize = request.url.substr(1);
    if (!msize) {
        msize = 1024;
    } else {
        msize = parseInt(msize);
    }
    if (!responses[msize]) {
        responses[msize] = Array(msize).join("X");
    }
    response.end(responses[msize]);
}


var server = http.createServer(handle);

server.listen(PORT, function() {
    console.log('Serving on ::' + PORT.toString());
});

--- 
+++ 
@@ -10,10 +10,13 @@
     if (!msize) {
         msize = 1024;
     } else {
-        msize = parseInt(msize);
+        msize = parseInt(msize, 10);
     }
     if (!responses[msize]) {
-        responses[msize] = Array(msize).join("X");
+	// for Node.js v4
+	responses[msize] = new Buffer(msize).fill("X");
+	// for Node.js v6
+	// responses[msize] = Buffer.alloc(msize, "X");
     }
     response.end(responses[msize]);
 }
Use Buffer with fill(), parseInt() with radix
