无法获取isNaN函数以使其正常工作

Question

I've created three text boxes. The first two text boxes are for two numbers (for subtraction). The result will appear in the third text box. I'm trying to make it so that if the user puts in a letter instead of a number, an alert happens.

Right now, the isNaN isn't working. Whenever I put anything in the first or second textbox, it alerts, regardless of number or letter. Here's my code.

function myFunction() {
    var x = document.getElementById("taxFree"); //textbox 1
    var y = document.getElementById("prodCost"); //textbox 2
    var res = document.getElementById("salesMargin"); //Result textbox

     // If x or y is Not a Number, send alert to user
    if (isNaN(x) || isNaN(y)) {
        alert("Numers only!");
        return;
    } 
    res.value = x.value - y.value;
}

Thanks in advance!

Answer 1

尝试检查input的value ：

if (isNaN(x.value) || isNaN(y.value)) {

Answer 2

The isNaN function tells you if a value equals to the special value NaN . If it is a string it will try to parse it befor. isNaN will not return true for every value that is not a number, for example isNaN(true) === false .

The other thing is that you try to call isNaN on the input element itself, not its value. Try this instead:

 function myFunction() { var taxFree = document.getElementById("taxFree"); var prodCost = document.getElementById("prodCost"); var salesMargin = document.getElementById("salesMargin"); //Result textbox var x = parseFloat(taxFree.value); var y = parseFloat(prodCost.value); // If x or y is Not a Number, send alert to user if (isNaN(x) || isNaN(y)) { alert("Numers only!"); return; } salesMargin.value = x - y; } 

 <input id="taxFree" value="15"> <input id="prodCost" value="10"> <input id="salesMargin" value="x"> <button onclick="myFunction()">Go!</button> 

Answer 3

I must say this method will not work if you want to know if the entered input is a number or not.

Try this , this function will only accept numbers from 0 to 9, nothing else at all even decimals not allowed. It does so by checking the entered input's char

code: onkeypress='return event.charCode >= 48 && event.charCode <= 57'

just add this line to your input as follows:

<input id="taxfree" onkeypress='return event.charCode >= 48 && event.charCode <= 57'>