Customizing 404 page

I have the following javascript code, I redirect old link to a new one dynamically.

Sometimes newLink does not exist, and instead of getting 404 page, I want to custom my own page if URL was not found.

Any idea how to add this condition in my code please ?

<script>
setTimeout(function(){
 $('a[href="oldLink"]').attr('href','dir/newLink)
 }, 1000);
</script>

Thanks !