Swap-Aggregator-Subgraph/node_modules/pull-stream/throughs/through.js

'use strict'

//a pass through stream that doesn't change the value.
module.exports = function through (op, onEnd) {
  var a = false

  function once (abort) {
    if(a || !onEnd) return
    a = true
    onEnd(abort === true ? null : abort)
  }

  return function (read) {
    return function (end, cb) {
      if(end) once(end)
      return read(end, function (end, data) {
        if(!end) op && op(data)
        else once(end)
        cb(end, data)
      })
    }
  }
}