JavaScript: Add thousands separators

float_separators.js

// add separators to any kind of a number
function sep(val) {
    if (!val) return val;
    
    var s = val + "",
        components = s.split("."),
        x = components[0].split("");
    
    for (var i = x.length - 3; i > 0; i -= 3) {
        x.splice(i, 0, ",");
    }
    
    return [x.join("")].concat(components.slice(1)).join(".");
}

// please note that JavaScript automatically removes trailing zeros to the right
// of a decimal point (e.g. sep(1000.0) -> "1,000")
// if you want to make sure your trailing zeros are left intact, pass the value
// as a string (e.g. sep("1000.0") -> "1,000.0")

sep(10000.5); // -> "10,000.5"
sep(123456.78); // -> "123,456.78"
sep(999.00); // -> "999"

integer_separators.js

// add separators to an integer
function sep(val) {
    var x = (val + "").split("");
    for (var i = x.length - 3; i > 0; i -= 3) {
        x.splice(i, 0, ",");
    }
    return x.join("");
}

sep(10000); // -> "10,000"
sep(123456); // -> "123,456"
sep(999); // -> "999"