试图用Javascript做Luhns算法，但我的编程一直在无限循环

Question

正如标题所说，我正在尝试用Javascript编写luhns算法，并让它打印出有效的卡号以及它是什么类型的卡，无论是美国运通卡、万事达卡还是Visa卡。

当运行时，它所做的只是不断地循环警报框。我想打印的是这是一个有效的卡片类型，以及它在警报框中是什么品牌的卡片。

function checkCreditCard() {
var ccnum = document.getElementById("cardnum").value;
var cardArray = [cardnum]
var temp = ccnum;
var checkerArray;
if (cardType(ccnum) === "AmericanExpress") {
checkerArray = [15];
for (x = 0; x < 15; x++) {
checkerArray[x] = ccnum % 10;
ccnum = ccnum / 10;
}
} else {
checkerArray = [16];
for (x = 0; x < 16; x++) {
checkerArray[x] = ccnum % 10;
ccnum = ccnum / 10;
if (ccnum == 0) {
checkerArray[15] = -1;
checkerArray[14] = -1;
checkerArray[13] = -1;
}
}
}
var summing;
for (x = 1; x < checkerArray.length; x = x + 2) {
if (checkerArray[x] < 0) {
return;
}
checkerArray[x] = checkerArray[x] * 2;
if (checkerArray[x] >= 10) {
summing = summing + checkerArray[x] % 10 + checkerArray[x] / 10
} else {
summing = summing + checkerArray[x];
}
}
for (x = 0; x < checkerArray.length; x = x + 2) {
if (checkerArray[x] < 0) {
return;
}
summing = summing + checkerArray[x];
if (summing == 20) {
alert("This Card is Legit")
} else {
alert("This Card is Invalid")
}
}
function cardType(ccnum) {
var x = {
Visa: /^4[0-9]{12}(?:[0-9]{3})?$/,
Mastercard: /^5[1-5][0-9]{14}$/,
AmericanExpress: /^3[47][0-9]{13}$/,
}
for (var l in x) {
if (x[l].test(ccnum)) {
return l;
}
}
return null;
}
}

<input type="text" id="cardnum" onkeyup="checkCreditCard()" />
<span id="valid">Enter a Number and Press Enter</span>

Answer 1

让我们使用Let

同时，让我们将警报移出循环

function cardType(ccnum) {
var x = {
Visa: /^4[0-9]{12}(?:[0-9]{3})?$/,
Mastercard: /^5[1-5][0-9]{14}$/,
AmericanExpress: /^3[47][0-9]{13}$/,
}
for (var l in x) {
if (x[l].test(ccnum)) {
return l;
}
}
return null;
}
document.getElementById("cardForm").addEventListener("submit", function(e) {
e.preventDefault();
var ccnum = document.getElementById("cardnum").value;
if (ccnum.length < 16) return
var cardArray = [cardnum]
var temp = ccnum;
var checkerArray;
if (cardType(ccnum) === "AmericanExpress") {
checkerArray = [15];
for (let x = 0; x < 15; x++) {
checkerArray[x] = ccnum % 10;
ccnum /= 10;
}
} else {
checkerArray = [16];
for (let x = 0; x < 16; x++) {
checkerArray[x] = ccnum % 10;
ccnum /= 10;
if (ccnum == 0) {
checkerArray[15] = -1;
checkerArray[14] = -1;
checkerArray[13] = -1;
}
}
}
var summing;
for (let x = 1; x < checkerArray.length; x = x + 2) {
checkerArray[x] *= 2;
if (checkerArray[x] >= 10) {
summing +=  checkerArray[x] % 10 + checkerArray[x] / 10
} else {
summing += checkerArray[x];
}
}
for (let x = 0; x < checkerArray.length; x = x + 2) {
summing += checkerArray[x];
}
if (summing == 20) {
alert("This Card is Legit")
} else {
alert("This Card is Invalid")
}
})

<form id="cardForm">
<input type="tel" id="cardnum" maxlength="16" />
<span id="valid">Enter a Number and Press Enter</span>
</form>