'use strict'

function id (e) { return e }
var prop = require('../util/prop')

module.exports = function map (mapper) {
  if(!mapper) return id
  mapper = prop(mapper)
  return function (read) {
    return function (abort, cb) {
      read(abort, function (end, data) {
        try {
        data = !end ? mapper(data) : null
        } catch (err) {
          return read(err, function () {
            return cb(err)
          })
        }
        cb(end, data)
      })
    }
  }
}