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) {
console.error(err)
return
}
console.log(Buffer.concat(data).toString())
}

const readers = files.map((file) => {
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.

You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (’).