$ is not defined - Ajax call in external js file

Question

I'm trying to execute a JavaScript function. I get an error everytime the function enters the Ajax block saying:

$ is not defined

How can I solve this problem? I'm Programming a Google Chrome Addon and I get this error in my .js file:

At the moment ther is just the function above and its caller in the file:

// Calls get_pw function
document.addEventListener('DOMContentLoaded', function(){
    document.getElementById('submit').addEventListener('click', get_pw)
})

Many Thanks

Answer 1

Include jQuery before executing the function or rewrite AJAX request without jQuery.

Including can be done simply by copying jQuery code at the beginning of your script or by loading a file from their CDN and then executing your script on a callback (slower).

Answer 2

Download jQuery from https://code.jquery.com/jquery-3.3.1.min.js . Include it in head tag of your document and call get_pw() when document has loaded.