flexible-counter.js

class FlexibleCounter {
  constructor(nums, op) {
    this.nums = nums;
    this.op = op;
    this.operatorWhiteList = ["+", "*", "/", "-"];
  }

  result() {
    if (this.operatorWhiteList.includes(this.op) && !this.nums.some(isNaN)) {
      return this.nums.reduce((total, el) => eval(`${total} ${this.op} ${el}`));
    } else {
      return "There was an issue with the data";
    }
  }
}

const sum = new FlexibleCounter([1, 2, 3], "+");
sum.result(); // 6

const product = new FlexibleCounter([1, 2, 3], "/");
product.result(); // 0.166

const badData = new FlexibleCounter(["Sneaky code", 2, 3], "+");
badData.result(); // "There was an issue with the data"