// Create a new mesh and assign it to the mesh filter
function Start () {
var mesh : Mesh = new Mesh ();
GetComponent(MeshFilter).mesh = mesh;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Start() {
Mesh mesh = new Mesh();
GetComponent<MeshFilter>().mesh = mesh;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Start():
mesh as Mesh = Mesh()
GetComponent[of MeshFilter]().mesh = mesh