Have you tried parseInt(), integers don't have leading 0s - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt

mobile = $('#country_code').val();
        mobile += parseInt(input.val());

You can use substring to remove the first N characters from a string.

First of all, assign a variable with the value of input.val()

let str = input.val()

Now, you can index the Nth element of str and check its value:

if (str[0] === "0") str = str.substring(1)

With this, you have successfully checked if the first character is "0" and reassigned the str variable accordingly.

For this, I'd use substring which returns a new string. Example :

mobile = '0652447766';
mobile = mobile.substring(1)
console.log('mobile = ', mobile)

>> mobile = '652447766'

Warning ! Calling mobile.substring() doesn't actually modify mobile ! Strings are immutable in Js, so you'll have to get the returned value of substring() with mobile = mobile.substring(1)

Hope it helped !