One line of javascript. What is this?

Its one of the unit tests for the “Arguments Optional” challenge in the Front End Development Certification course. Pretty standard, the student writes the body of a function called by the unit test.

Last unit test function call looks like this:


What is the second paren? Looks like a syntax error to me, but the interpreter doesnt mind. What am I missing?

This syntax is valid. This is called currying. Basically the function returned from addTogether(2) is immediately called with [3] as an argument.