Learning Redux

Functions as arguments

The first type of higher-order functions are functions that take other functions as arguments:

function someFn (fn) { ... }

You might be familiar with this from the callback pattern, which used to be very common when working with asynchronous JavaScript. We pass a callback function that takes an error object as the first argument and the data as the second. If no error happens, the first argument will be null:

function requestAndDo (callback) {  return fetch('http://localhost:1234')    .then(data => callback(null, data))    .catch(err => callback(err))}

A function like this can be called by passing a callback function to it:

requestAndDo((err, data) => {  if (err) throw err  // ...handle data...})

Similarly, we could write a ...