Mastering JavaScript Functional Programming - Second Edition

An unnecessary mistake

There is, however, a common (though in fact, harmless) mistake that is usually made. You often see code like this:

fetch("some/remote/url").then(function(data) {  processResult(data);});

What does this code do? The idea is that a remote URL is fetched, and when the data arrives, a function is called—and this function itself calls processResult with data as an argument. That is to say, in the then() part, we want a function that, given data, calculates processResult(data). But don't we already have such a function?

A small bit of theory: in lambda calculus terms, we are replacing λx.func x with simply func—this is called an eta conversion, or more specifically, an eta reduction. (If you were to do it the other way round, ...