"unit" in unit test in JavaScript

I have been working on Jasmine from the last one year.
Apart from that, I read lot of blogs to understand the concept of "Unit testing" and "Test Driven Development"
Every time I read, I get this doubt. What actually "unit" we write. What could be granularity? In specific to JavaScript, is it a Statement or a function ?
Definitely from my experience, I could say it as function.
If I have to give a definition to a fresher who has no knowledge of unit testing What could I say? (specific to JavaScript & Jasmine is fine)
Thanks!