JS thousandth algorithm implementation ideas_javascript skills

Copy code The code is as follows:

function commafy() {
var num = document.getElementById( "NumA").value;
//1. First remove the spaces and determine whether it is a null value or a non-number
num = num "";
num = num.replace(/[ ]/g, " ");
if (num == "") {
alert("Null value, end");
return;
}
if (isNaN(num)) {
alert("Not a number, end");
return;
}
//2. Depending on whether there is a decimal point, handle it according to the situation
var index = num.indexOf(".");
if (index==-1) {//No decimal point
var reg = /(-?d )(d{3})/;
while (reg.test(num)) {
num = num.replace(reg, "$1,$2");
}
} else {
var intPart = num.substring(0, index);
var pointPart = num. substring(index 1, num.length);
var reg = /(-?d )(d{3})/;
while (reg.test(intPart)) {
intPart = intPart. replace(reg, "$1,$2");
}
num = intPart "." pointPart;
}
return alert(num);
}