Mapping Array in Javascript with sequential numbers

The following code: Creates the following Array: I just don’t understand why. I can’t find anything on the internet that explains this behavior. Does anyone know why this works the way it does? Perhaps a link to some documentation? Answer creates an array of length 10 with all elements being undefined. will invoke for each element with the arguments (element, index, array) and setting this to Number. The first argument to call will be taken as this (not relevant here), and all the other arguments are passed as they are, with the first one being the index. And Number

reduce function composed of map function in JavaScript

Say we have and want to reduce() it like Now, I want to compose reduce() function itself from map() function. How would you do that? What is the smartest way? Edit: In the comments and answers, many have claimed fold/reduce can compose map, in shallow level, that can be true, however, in category theory, fundamentally reduce/fold is generalized to Catamorphism and it’s all about functors (map), and it’s called F-algebra. Answer If you’re trying to build the map function using reduce, you could do the following (The example I’m providing will use built-in functions and work for arrays

What is the difference between .map, .every, and .forEach?

I’ve always wondered what the difference between them were. They all seem to do the same thing…