简单的晚餐小费计算器的Javascript If语句

Question

I have this script that prompts for dinnerCost and taxRate to generate taxPaid. I want to make an if statement that makes sure that only integers are entered not strings.

here is my code:

<button id="btn"> Play the Dinner Game </button>

<p id="para"></p>



document.getElementById("btn").addEventListener("click", dinnerThing);



function dinnerThing () {
        var cost = prompt("how much did it cost?");

        if(cost == "") {
            document.getElementById("para").innerHTML = "sorry please enter an integer";    
        }

        var taxRate = prompt("What was the tax rate?");

        if(taxRate == "") {
            document.getElementById("para").innerHTML = "sorry please enter an integer";    
        }



        var taxPaid = cost * taxRate;


        document.getElementById("para").innerHTML = "You paid " + taxPaid + " dollars in tax for your dinner";
}

Why is if statement not working here?

Answer 1

Both if(cost == "") and if(taxRate == "") are checking if the variable ( cost1 or taxRate`) is equal to an empty string, not if it is equal to an integer .

To ensure that you receive a value from prompt that is an Number you can use parseInt() .

parseInt() will return a Number (integer or float) equivalent from a string. If the string cannot be converted into a number, it returns NaN.

The steps would then be:

• get parseInt() values for cost and taxRate, store them in variables

• check if either of the previous variables returns is NaN using isNaN() .

• If isNaN() returns true , then display "sorry please enter an integer" message

• If isNaN() returns false , then calculate taxPaid and output taxPaid value

The resulting code would look something like this:

var numberCost = parseInt(cost),
  numberTaxRate = parseInt(taxRate);

if(isNaN(numberCost) || isNaN(numberTaxRate)) {
   document.getElementById("para").innerHTML = "sorry please enter an integer";  
}else{
  var taxPaid = cost * taxRate;
  document.getElementById("para").innerHTML = "You paid " + taxPaid + " dollars in tax for your dinner";
}