ncu-exam.js

    // This would provide `createFile`, `createReadStream`, `writeContent` and `createWriteStream`.
    var FileSystem = require('FileSystem');
    var Reader = function() {
      this.counter = 0;
    };
    Reader.prototype = FileSystem.prototype;
    Reader.prototype.paths = ['/tmp/urandom-1.txt', '/tmp/urandom-2.txt'];
    Reader.prototype.start = function() {
      this.mainPromise = Promise.resolve()
      .then(function() {
        this.urandomStream = this.createReadStream('/dev/urandom');
        this.urandomStream.setEncoding('hex');
        this.urandomStream.on('data', this.onChunk);
      });

      // Returning it to keep the method could be tested.
      return this.mainPromise;
    };

    Reader.prototype.onChunk = function(chunk) {
      if (this.counter && this.counter ===  5) {
        this.urandomStream.close();
        this.targetStream.close();
        return;
      }
      this.createFile();
      this.counter += 1;

      this.mainPromise
      .then(this.writeContent(chunk))
      .catch(function(error) {
        console.error(error);
      });
    };

    Reader.prototype.createFile = function() {
      if (0 === this.counter) {
        this.filePath = this.paths[0];
      } else if(2 === this.counter) {
        this.filePath = this.paths[1];
      }
      
      this.targetStream = this.createWriteStream(this.filePath);
      return Promise.resolve();
    };

    Reader.prototype.writeContent = function(chunk) {
      return function() {
        return new Promise(function(resolve, reject) {
          this.targetStream.write(chunk, 'utf8', resolve);
        });
      };
    };

    var reader = new Reader();
    reader.start();