Anwser to Q3 - ( only doing 3.1 Sum )

csv-reducer.js

const CsvReader = require('./csv-reader');

// reusing what we've done before.
const exisMath = {
  sum (...args) {
    return args.reduce((acc, i) => {
      return parseFloat(acc) + parseFloat(i)
    }, 0)
  },
}

class CsvReducer {
  constructor (stream, { reduce: reduceMethod }) {
    this.stream = stream
    this.reduceMethod = reduceMethod
    this.reader = new CsvReader(stream);

    const [column, reducer] = Object.entries(this.reduceMethod)[0];
    this.column = column;
    this.reduceFn = exisMath[reducer]
  }

  async reduce () {
    const output = {
      [this.column]: 0
    }
    this.reader.on('row', row => {
      output[this.column] = this.reduceFn(output[this.column], row[this.column])
    })
    await this.reader.done();
    return output
  }
}

module.exports = CsvReducer