未加载库时如何调试参考错误？

Question

For example I am getting a reference error L is not defined where L is leaflet. I have followed their tutorial , and here is what I have just before the end of my body:

<script src="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.js"></script>
<script>
    var map = L.map('map').setView([51.505, -0.09], 13);
    L.tileLayer('http://{s}.tiles.mapbox.com/v4/secretid/{z}/{x}/{y}.png', {
      attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
      maxZoom: 18
    }).addTo(map);
</script>

I've checked and the hyperlink to the js file works.

Answer 1

To be sure all script have already loaded, use onload callback like this:

<script>
    window.onload = function() {
     var map = L.map('map').setView([51.505, -0.09], 13);
     L.tileLayer('http://{s}.tiles.mapbox.com/v4/secretid/{z}/{x}/{y}.png', {
attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a     href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a     href="http://mapbox.com">Mapbox</a>',
 maxZoom: 18
    }).addTo(map);
}
</script>