Please explain callback?

  1. what is fastseries and cb,
const { readFile } = require('fs')
const series = require('fastseries')()
const files = Array.from(Array(3)).fill(__filename)

const print = (err, data) => {
if (err) {

const readers = => {
return (_, cb) => {
readFile(file, (err, contents) => {
if (err) cb(err)
else cb(null, contents)

series(null, readers, null, print)

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

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See this post to find the backtick on your keyboard.
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