Editable calculator on plain JS

calc.js

function Calculator() {
  this.operations = {
    '+': (a, b) => +a + +b,
    '-': (a, b) => a - b,
  };
  
  this.validateInputNumbers = function(arr) {
    return !(isNaN(arr[0]) || isNaN(arr[2]))
  };
  
  this.validateInputOperator = function(arr) {
    return Object.keys(this.operations).includes(arr[1])  
  };
 
  this.calculate = function(str) {
    let arr = str.split(' ');

    if (!this.validateInputNumbers(arr)) {
      return 'Inputted numbers are incorrect'
    }
    if (!this.validateInputOperator(arr)) {
      return 'Inputted operator is unknown'
    }

    let operator = arr.splice(1, 1);

    return arr.reduce(this.operations[operator])  
  };

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

let calc = new Calculator();
console.log(calc.calculate('1 + 2'))
console.log(calc.calculate('2 + 2'))
console.log(calc.calculate('6 - 2'))
console.log(calc.calculate('6 - 10'))
console.log(calc.calculate('6s - 2'))
console.log(calc.calculate('6 - s2'))
console.log(calc.calculate('6- 2'))
console.log(calc.calculate('6 2'))
console.log(calc.calculate('6 -2'))
console.log(calc.calculate('lol12'))
console.log(calc.calculate('6 * 2'))
calc.addMethod("*", (a, b) => a * b);
console.log(calc.calculate('6 * 2'))