Mastering JavaScript Functional Programming - Second Edition

Working with arguments

In Chapter 1, Becoming Functional - Several Questions, and Chapter 2, Thinking Functionally - A First Example, we saw some uses of the spread (...) operator. However, the most practical usage we'll be making of it has to do with working with arguments; we'll see some cases of this in Chapter 6, Producing Functions – Higher-Order Functions. Let's review our once() function:

const once = func => {  let done = false;  return (...args) => {    if (!done) {      done = true;      func(...args);    }  };};

Why are we writing return (...args) => and then afterwards func(...args)? The answer has to do with the more modern way of handling a variable number (possibly zero) of arguments. How did you manage such kinds of code in older versions of ...