Add Prefix in text field

When you have to prefix any hard coded value in a text field, like for example in telephone field you have to prefix the country code and dont want the user to remove it, then in that case assign the prefix code in the text field and add the below javascript code code

jQuery("input[name='job-code']").keydown(function(e) {
    var oldvalue = jQuery(this).val();
    var field = this;
    setTimeout(function() {
        if (field.value.indexOf('JPC-') !== 0) {
            jQuery(field).val(oldvalue);
        }
    }, 1);
});

Demo: https://codepen.io/phpweblamp/pen/yLwPqjJ