Center object STl

I'm using Three Js to render and display an STL object in the browser from a file-type input.

window.addEventListener("load", function () {
"use strict";

var w = 400, h = 300;

var renderer = new THREE.WebGLRenderer({ alpha: true });
renderer.setSize(w, h);
//renderer.setClearColorHex( 0x000000, 1 );
var view = document.getElementById("view");
view.appendChild(renderer.domElement);

var camera = new THREE.PerspectiveCamera(45, w / h, 1, 1000);
camera.position.set(0, 0, 50);

var controls = new THREE.TrackballControls(camera, view);

var scene = new THREE.Scene();
scene.add(new THREE.AmbientLight(0x666666));

var light1 = new THREE.DirectionalLight(0xffffff);
light1.position.set(0, 100, 100);
scene.add(light1);

var light2 = new THREE.DirectionalLight(0xffffff);
light2.position.set(0, -100, -100);
scene.add(light2);

var mat = new THREE.MeshPhongMaterial({
    color: 0x64DD17, ambient: 0x64DD17, specular: 0x64DD17,
});
var obj = new THREE.Mesh(new THREE.Geometry(), mat);
scene.add(obj);

var loop = function loop() {
    requestAnimationFrame(loop);
    //obj.rotation.z += 0.05;
    controls.update();
    renderer.clear();
    renderer.render(scene, camera);
};
loop();

// file load
var openFile = function (file) {
    var reader = new FileReader();
    reader.addEventListener("load", function (ev) {
        var buffer = ev.target.result;
        var geom = loadStl(buffer);
        scene.remove(obj);
        obj = new THREE.Mesh(geom, mat);
        scene.add(obj);
    }, false);
    reader.readAsArrayBuffer(file);
};

// file input button
var input = document.getElementById("file");
input.addEventListener("change", function (ev) {
    var file = ev.target.files[0];
    openFile(file);
}, false);

// dnd
view.addEventListener("dragover", function (ev) {
    ev.stopPropagation();
    ev.preventDefault();
    ev.dataTransfer.dropEffect = "copy";
}, false);
view.addEventListener("drop", function (ev) {
    ev.stopPropagation();
    ev.preventDefault();
    var file = ev.dataTransfer.files[0];
    openFile(file);
}, false);
}, false);

However when I load an object, it does not come centered on the screen, forcing the user to zoom in or out according to the size of the object.

HowcanIconfigureThreeJstocentralizethisobjectautomatically?