Example of client side javascript testing

If I have js-file /static/js/myfile.js with content like:

function foo() {...}

How can I test this function (or, in a broader sense, any functions in any particular files)? E.g. using karma or anything else...

It's question about unit-tests. I'm pretty new with testing and a large number of different tools a little confusing. So I just asking about some sort of example.