bookmarklet_example.js

if (!($ = window.jQuery)){ // typeof jQuery=='undefined' works too  
    script = document.createElement( 'script' );  
    script.src = 'http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js';   
    script.onload=dosomething;
    document.body.appendChild(script);  
}   
else{  
    dosomething();
}  
      
function dosomething(){  
}