How to make JavaScript work when button is pressed?

I am a bit of a newbie in JS and not too sure what I'm doing.

I have created a button

<div id="btn">
    <a href='dg-quiz-maker.js' class='button'>ATTEMPT A QUIZ</a>
</div>

I want when the user presses the button, the JS dg-quiz-makes.js file appears. I'm not sure how to I do it. Please help.