pass json data into javascript

Hello from a Ruby on Rails beginner!

I've been trying to pass some data to a variable in Javascript by using
json. Seems like the Javascript variable doesn't receive the data

This is the code in controller. @courses variable find the data from

class CoursesController < ApplicationController

  def index
    @courses = Course.find(:all)

This is Javascript code in view.

<script type="text/javascript">
var markers=<%= @courses.to_json %>;

The following code show the data in browser with no problem. That means

<p><%= @courses.to_json %></p>

But the following code doesn't show data in browser right. The output is
a string of "undefined".

<script type="text/javascript">
var markers = <%= @courses.to_json %>;
for(i=0; i<markers.length; i++) {
document.write(markers[i].latitude, markers[i].longitude,

I don't know what goes wrong. Please help. Thank you in advance.