How to change the content of a variable to zero if isNaN [duplicate]

Answer 1

You variables are not in scope when the isNaN() function attempts to check them.

Answer 2

your formatting is messed up and caused you to exit the function fixOrder before it really did anything:

function fixOrder() {
    var numPrice = parseFloat(document.getElementById("cost").value);
    var taxP = parseFloat(document.getElementById("tax").value);
    var total = parseFloat(document.getElementById("total").value);



    if (isNaN(numPrice)) {
        alert("Sorry,you must enter a numeric value to place order");
        return; //gets syntax error
    }

    if (isNaN(taxP)) {
        alert("Sorry, you must enter a numeric tax value to continue");
        return;
    }

    var tax = (numPrice * taxP) / 100;
    var total = numPrice + tax;
    numPrice = 0;
    taxP = 0;
    document.getElementById("total").value = "$" + total.toFixed(2);
    return; //gets syntax error
}

See how much easier to read that is? Where you original code could be reduced to :

document.getElementById("total").value = "$NaN";