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.)

In the first example the genPrimes function is being created 2e4 times, once for each iteration of the loop. In the second example it's defined once.

In the first example, he creates a unique closure per point instance and places it on the point itself.

In the second example, he creates a single closure and places it on the point's prototype.

Unsurprisingly, creating one object is faster than creating a thousand of them. I'm not sure why the author thinks this has anything to do with closures.

This is in general a good advice for most languages; instead of calling a function multiple times, do it once (and potentially store the result if you want it for later use). A similar, albeit far more simple, example would be calling a length method when iterating over an array instead of saving it to a variable.