Javascript object-style module to functional programming

Question

I'm working on a simple Javascript module and I'd like to make it follow the functional programming style.

This module basically allows you to measure how much time has passed when you started and finished an event and sends the data in chunks of 2 measurements:

const register = {
  marks: {},
  marksCount: 0,

  restart: function () {
    this.marks = {}
    this.marksCount = 0
  },

  startMark: function (id) {
    performance.mark(`${id}/start`)
  },

  finishMark: function (id) {
    performance.mark(`${id}/end`)
    performance.measure(id, `${id}/start`, `${id}/end`)
    this.marksCount++
    this.marks[id] = performance.getEntriesByName(id, 'measure')[0]
    if (this.marksCount === 2) {
      console.log(this.marks)
      this.restart()
    }
  }
}


// then you can use it like this
register.startMark('event1')
register.startMark('event2')
register.finishMark('event1')
register.finishMark('event2')

I've been reading some posts regarding how FP manages the state and I'd really like to see how this simple module can be written using pure FP principles, specially how can we prevent mutating the properties.

If it makes sense to use a stateless object, use a stateless object. there is no sense in blindly applying paradigms — Jonas Wilms
– Jonas Wilms, Commented Oct 9, 2018 at 21:30
In any case if you know what the Functional Programming Paradigm is then you should try to apply it on your own. If you have trouble from that aspect then I'm sure the stackoverflow community could be of more help - or perhaps more readily, the codereview community — zfrisch
– zfrisch, Commented Oct 9, 2018 at 21:32
I tried but did not found a proper way to make the properties immutable. — Emi
– Emi, Commented Oct 9, 2018 at 21:35

Sylwester · Accepted Answer · 2018-10-09 23:06:41Z

2

It's very simple to make an object inaccessible for write. eg.

const storage = v => () => v;
const test = storage(10); 
console.log(test()); // prints 10, but you have no way to change v

A functional version of your time measure might look like this:

const startMark = () => {
  const start = performance.now();
  return () => {
    const end = performance.now();
    return end - start;
  };
}

// How to use
const endMark1 = startMark();
const endMark2 = startMark();
const result1 = endMark1();
const result2 = endMark2();
console.log('time1', result1, 'time2', result2);

// do something else
const result11 = endMark1();
console.log('time1-2', result11);

Since it is functional you can use the endTimer several times and it will return the time from when it started to now even on consecutive calls.

answered Oct 9, 2018 at 23:06

Sylwester

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1 Comment

Emi Over a year ago

Very interesting approach. I'm just wondering how can we add the marksCount logic without storing it somewhere.

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