How to change the :before content of #abc style from javascript ?
<script>
document.getElementById('abc').style.content='-';
</script>
<style>
#abc:before{content:+;}
</style>
No, you cannot access :before or :after from javascript, because they are not a part of the DOM. However you can still achieve your goal by using CSS classes:
<script>
document.getElementById('abc').className = minus;
</script>
<style>
#abc:before {content: +;}
#abc.minus:before {content: -}
</style>
In fact this approach is more unobtrusive, because you don't mix representation with javascript. Tomorrow you might want to change text +/- to say nice background images, in this case you don't have to touch javascript code at all.
