My methodology for avoiding this confusing “functional” programming:

1. Write:

  function add (a, b) { return a + b; }

2. Refactor to object:

  class AccumulatingIntegerWidget {
    constructor (...numbers) {
      this.accumulation = 0;
      this.accumulate(...numbers);
    }
    accumulate (...numbers) {
      for (const n of numbers) {
        this.accumulation = this.accumulation + n;
      }
    }
    getAccumulation () {
      return this.accumulation;
    }
  }

Presto! It’s stateful and object-oriented and handles future cases like adding three numbers, even though WeArentGonnaNeedIt!

If this does not satisfy, I progress to making my classes observe each other and update themselves automatically. “We’re doing two-way binding!” I exclaim proudly, while leaving debugging as an exercise for my colleagues.