How to convert string to number or integer in js?

var str = "123";
alert(str+"---"+typeof(str));
str = Number(str);
alert(str+"---"+typeof(str));

How to find variable type in js & jquery?

var str = "string";
alert(typeof(str)); //output: string

Input field enter integer only in jQuery

$(document).ready(function () {
    //called when key is pressed in textbox
    $("#quantity").keypress(function (e) {
        //if the letter is not digit then display error and don't type anything
        if (e.which != 8 && e.which != 0 && (e.which < 48 || e.which > 57)) {
            //display error message
            $("#errmsg").html("Digits Only").show().fadeOut("slow");
            return false;
        }
    });
});

Number :
<input type="text" name="quantity" id="quantity" /> <span id="errmsg"></span>