SOURCE

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

  this.addMethod("+", (a, b) => a + b);
  this.addMethod("-", (a, b) => a - b);
  this.addMethod("*", (a, b) => a * b);
  this.addMethod("/", (a, b) => a / b);
  this.addMethod("**", (a, b) => a ** b);  

  this.isOperator = function (item) {
    return isNaN(Number(item)) ? true : false;
  };

  this.priority = {'+' : 1, '-' : 1, '*' : 2, '/' : 2, '**':3};

  this.isValidOper = function (op) {
    if (op in this.priority) {
        return true;
    } else {
        return false;
    }
  };

  this.opCmp = function(op1, op2) {
    return +this.priority[op1] - (+this.priority[op2]);
  };

  this.calculate = function (str) {
    if (!str) {
        return;
    }

    let arr = str.split(' ');
    let operators = [];
    let num = [];
    let top = '';
    while (arr.length != 0) {
        top = arr.pop();
        console.log("arr top: " + top);
        if (this.isOperator(top)) {
            if (operators.length != 0) {
                if (this.opCmp(top, operators[operators.length-1]) <= 0) {
                    let op = operators.pop();
                    let a = num.pop();
                    let b = num.pop();
                    num.push(this[op](+a, +b));
                    arr.push(top);
                } else {
                    operators.push(top);
                }
            } else {
                operators.push(top);
            }
            
        } else {
            num.push(top);
        }

        console.log("op: " + operators);
        console.log("num: " + num);
    }

    while (operators.length != 0) {
        let op = operators.pop();
        let a = num.pop();
        let b = num.pop();
        num.push(this[op](+a, +b));
    }

    return num.pop();
  };
}

let powerCalc = new Calculator;
let result = powerCalc.calculate("1 + 2 ** 3 * 2");
console.log( result ); // 8

result = powerCalc.calculate("1 + 2 ** 3 * 4 + 2");
console.log( result ); // 8