"readLine" and "parseInt" are they frontend or backend

Answer 1

Neither frontend nor backend both are function of JavaScript. readLine is a function in Node JS which is used to take user input.

parseInt is a function which is used to parse user input to integer.

readLine by default returns string so if you want to take number then you have to use parseInt

var num = parseInt(readLine())

Answer 2

They're ain't actually associated to anything but readline is a nodejs inbuilt module for taking input (only limited to nodejs) however parseInt works on both browser & nodejs.