Fork from https://learn.javascript.ru/ Создать расширяемый калькулятор

<!doctype html>
<script src="https://ru.js.cx/test/libs.js"></script>
<script src="test.js"></script>
<script>

  const Calculator = function () {
  
  this.methods = {
    '+': (a, b) => a + b,
    '-': (a, b) => a - b
  };

  this.calculate = function (str) {
    
    const splitStr = str.split(' ');
    let a = parseInt(splitStr[0], 10);
    let opr = splitStr[1];
    let b = parseInt(splitStr[2], 10);

    if (!this.methods[opr] || !isFinite(a) || !isFinite(b)) {
     return NaN;
    }

    return this.methods[opr](a, b);
  };

  this.addMethod = function (name, func) {
    this.methods[name] = func; 
  };
};

let calc = new Calculator;
console.log( calc.calculate('3 + 8') );

let powerCalc = new Calculator;
powerCalc.addMethod('*', (a, b) => a * b);
powerCalc.addMethod('/', (a, b) => a / b);
powerCalc.addMethod('**', (a, b) => a ** b);

let result = powerCalc.calculate('2 ** 3');
console.log(result);

</script>

</html>

describe("Calculator", function() {
  let calculator;

  before(function() {
    calculator = new Calculator;
  });

  it("calculate(12 + 34) = 46", function() {
    assert.equal(calculator.calculate("12 + 34"), 46);
  });

  it("calculate(34 - 12) = 22", function() {
    assert.equal(calculator.calculate("34 - 12"), 22);
  });

  it("add multiplication: calculate(2 * 3) = 6", function() {
    calculator.addMethod("*", (a, b) => a * b);
    assert.equal(calculator.calculate("2 * 3"), 6);
  });

  it("add power: calculate(2 ** 3) = 8", function() {
    calculator.addMethod("**", (a, b) => a ** b);
    assert.equal(calculator.calculate("2 ** 3"), 8);
  });
});