In the under-application case, the remaining parameters get assigned the undefined value. In the over-application case, the remaining arguments can be accessed by using the rest parameter and the arguments property, or they are simply superfluous and they can be ignored. Many Web/NodeJS frameworks nowadays use this JS feature to accept optional parameters and create a more flexible API.
Until recently, V8 had a special machinery to deal with arguments size mismatch: the arguments adaptor frame. Unfortunately, argument adaption comes at a performance cost, but is commonly needed in modern front-end and middleware frameworks. It turns out that, with a clever trick, we can remove this extra frame, simplify the V8 codebase and get rid of almost the entire overhead.