load jQuery in another js file

Question

I have a JavaScript file, which also uses jQuery in it too. To load it, I wrote this code:

function include(filename)
{
    var head = document.getElementsByTagName('head')[0];
    var script = document.createElement('script');
    script.src = filename;
    script.type = 'text/javascript';
    head.appendChild(script)
}

include('http://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js');
alert("1");

$(document).read(function(){});
alert("2");

This fires alert("1"), but the second alert doesn't work. When I inspect elements, I see an error which says that $ in not defined.

How should I solve this problem?

Answer 1

You need to execute any jQuery specific code only once the script is loaded which obviously might happen at a much later point in time after appending it to the head section:

function include(filename, onload) {
    var head = document.getElementsByTagName('head')[0];
    var script = document.createElement('script');
    script.src = filename;
    script.type = 'text/javascript';
    script.onload = script.onreadystatechange = function() {
        if (script.readyState) {
            if (script.readyState === 'complete' || script.readyState === 'loaded') {
                script.onreadystatechange = null;                                                  
                onload();
            }
        } 
        else {
            onload();          
        }
    };
    head.appendChild(script);
}

include('http://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js', function() {
    $(document).ready(function() {
        alert('the DOM is ready');
    });
});

And here's a live demo.

You may also take a look at script loaders such as yepnope or RequireJS which make this task easier.