//計算加減乘除   
function calulate(firstNumber, secondNumber, symbol) {
symbol = symbol || '+'
let firstDecimal = firstNumber.toString().indexOf('.') != -1 ? firstNumber.toString().split('.')[1].length : 0
let secondDecimal = secondNumber.toString().indexOf('.') != -1 ? secondNumber.toString().split('.')[1].length : 0
let maxDecimal = secondDecimal > firstDecimal ? secondDecimal : firstDecimal
let result = 0
let firstInt = parseInt(firstNumber.toString().replace('.','')) * Math.pow(10, (maxDecimal-firstDecimal))
let secondInt = parseInt(secondNumber.toString().replace('.','')) * Math.pow(10, (maxDecimal-secondDecimal))
switch (symbol) {
    case '+':
        result = firstInt + secondInt
        return result / Math.pow(10, maxDecimal)
        break
    case '-':
        result = firstInt - secondInt
        return result / Math.pow(10, maxDecimal)
        break
    case '*':
        result = firstInt * secondInt
        return result / Math.pow(10, maxDecimal*2)
        break
    case '/':
        return result = firstInt / secondInt
        break
    default:
        result = firstInt + secondInt
        return result / Math.pow(10, maxDecimal)
        break
}
}