How does this js know $ get is function (id) {return document.getElementById (id); }

So, I was looking at a blog post with some javascript in it and there is something I am looking at that doesn't quite follow:

function pageLoadedHandler(sender, args) {  
    if (typeof(lastFocusedControlId) !== "undefined" 
         && lastFocusedControlId != "")   
    {  
        var newFocused = $get(lastFocusedControlId);  
        if (newFocused) {    
            focusControl(newFocused);
        }
    }

      

        
        
        
      

    

In the above method, it calls $ get which I am assuming to be an alias

 function(id) { 
     return document.getElementById(id); 
 }

      

        
        
        
      

    

The supplied js file does not contain the $ get argument.

This is a reserved alias and someone might provide a link that confirms this. If not, how does he know what $ get is?