Question

I am trying to include jQuery to an HTML page conditionally. It only needs to be added if it doesn’t exist yet.

I am using the following code near the top of my body tag to inject a script tag that includes the jQuery library in the head.

<script type="text/javascript">

    if (typeof jQuery === 'undefined') {
        alert('now adding jquery');
        var head = document.getElementsByTagName("head")[0];
        script = document.createElement('script');
        script.type = 'text/javascript';
        script.src = 'https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js';
        head.appendChild(script);
        if (typeof jQuery === 'undefined') {
            alert('jquery still not present :(');
        }
    } else {
        alert('jquery already present');
    }
</script>

When I execute it, I get the message that jQuery is still not present after adding it. The script tag does correctly show up in the loaded page’s source.

Trying to make use of jQuery a little further down below in my page, confirms that jQuery is indeed not working. As expected, Chrome’s JavaScript console says ‘$ not defined’.

How can I get this to work?

Answer 1

A script loads asynchronously. This means its contents are not directly available after appending the element.

Use onload instead:

script = document.createElement('script');

script.onload = function() {
    // jQuery is available now
};

script.type = 'text/javascript';
script.src = 'https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js';

head.appendChild(script);