playlist.js

const assert = require('assert')

class Playlist {

  constructor(items) {
    this.__items = items
    this.__index = 0
  }

  isLastItem() {
    return (this.__index === this.__items.length - 1)
  }

  isFirstItem() {
    return (this.__index === 0)
  }

  current() {
    return this.__items[this.__index]
  }

  outOfRange(index) {
    return index < 0 || index >= this.__items.length
  }

  setCurrent(index) {
    if (this.outOfRange(index))
      throw new Error('Out of range')

    return this.__index = index
  }

  next() {
    if (this.isLastItem())
      this.__index = 0
    else
      this.__index = this.__index + 1

    return this
  }

  previous() {
    if (this.isFirstItem())
      this.__index = this.__items.length
    else
      this.__index = this.__index - 1

    return this
  }

}

const playlist = new Playlist(['a', 'b', 'c'])

assert.equal(playlist.current(), 'a')

playlist.next()
assert.equal(playlist.current(), 'b')

assert.equal(playlist.next().current(), 'c')

playlist.setCurrent(0)
playlist.setCurrent(2)

try {
  playlist.setCurrent(-1)
} catch (err) {
  assert.equal(err.message, 'Out of range')
}