JavaScript functions have identity, so creating anonymous functions, even if they don't close over anything, actually requires a garbage-collected object to be

JavaScript functions have identity, so creating anonymous functions, even if they don't close over anything, actually requires a garbage-collected object to be allocated, rather than just creating a pointer to a piece of code. Doing this in inner loops is going to cause a noticeable performance degradation. (Doing it in expensive asynchronous operations, which is what callbacks are often used for, is not going to make a measurable difference.)