In javascript this refers to the object you call a function on. But since a function is also an object in javascript you can do this().

Here this refers to the function you call pipe on, so you call the original function with the arguments and pass the result to the piped function as an argument so addOne.pipe(square)(1) is actually square(addOne(1)) where this refers to addOne

Not sure what you mean by asynchronous code, but this solution definitely slows down considerably as the list lengthens. And yes, using a for loop that returns as soon as it finds its target, you'll return the answer more or less instantly if that target is near the beginning, even on lists millions of ints long. Not so for this 'top' solution.

This isn't async code, this is functional programing.

Even more. AP is a sorted list, so we can use the binary search to find a missed element