find-usb-printer.ts

import fs from 'fs'

/**
 * Determines the maximum lp that will be tested.
 * @example MAX = 10 // => '/dev/usb/lp9'
 */
const MAX = 10

/**
 * How much to wait after each testing cycle.
 */
const WAIT = 5000

const write = (path: string, data: any) => {
  return new Promise((resolve, reject) => {
    fs.writeFile(path, data, err => {
      if (err) {
        reject(err)
      } else {
        resolve()
      }
    })
  })
}

/**
 * Tries to write to every /dev/usb/lp from 0 to MAX. Returns on success.
 */
export const findInterface = async (): Promise<string> => {
  return new Promise(async resolve => {
    while (true) {
      for (let i = 0; i < MAX; i++) {
        const iface = `/dev/usb/lp${i}`

        try {
          await write(iface, '')
          // Got it!
          console.log(`${iface} will be used`)
          return resolve(iface)
        } catch (err) {
          console.log(`${iface} not available`)
        }

        // Wait a bit when just cycled through all interfaces
        const isLast = i === MAX - 1
        if (isLast) {
          await new Promise(resolve => setTimeout(resolve, WAIT))
        }
      }
    }
  })
}