1  'use strict';
 2  
 3  const { PassThrough } = require('stream');
 4  
 5  module.exports = function (/*streams...*/) {
 6    var sources = []
 7    var output  = new PassThrough({objectMode: true})
 8  
 9    output.setMaxListeners(0)
10  
11    output.add = add
12    output.isEmpty = isEmpty
13  
14    output.on('unpipe', remove)
15  
16    Array.prototype.slice.call(arguments).forEach(add)
17  
18    return output
19  
20    function add (source) {
21      if (Array.isArray(source)) {
22        source.forEach(add)
23        return this
24      }
25  
26      sources.push(source);
27      source.once('end', remove.bind(null, source))
28      source.once('error', output.emit.bind(output, 'error'))
29      source.pipe(output, {end: false})
30      return this
31    }
32  
33    function isEmpty () {
34      return sources.length == 0;
35    }
36  
37    function remove (source) {
38      sources = sources.filter(function (it) { return it !== source })
39      if (!sources.length && output.readable) { output.end() }
40    }
41  }