How add class checked for type=checkbox?

I'm trying to add a class "checked", but for some reason nothing happens. Here is the code snippet. Platform - Wordpress. Plugin - WC Fields Factory. Here is the broader part of the code, since you need to enable the changes here. Maybe I'm wrong.

this.doValidate = function( field ) {           
    if( field.attr("wccpf-type") !== "radio" && field.attr("wccpf-type") !== "checkbox" && field.attr("wccpf-type") !== "file" ) {
        if(field.attr("wccpf-type") !== "select") {
            if( this.doPatterns( field.attr("wccpf-pattern"), field.val() ) ) {                     
                field.next().hide();
            } else {                        
                this.isValid = false;
                field.next().show();
            }
        } else {
            if (field.val() !== "" && field.val() !== "wccpf_none") {
                field.next().hide();
            } else {
                this.isValid = false;
                field.next().show();
            }
        }                           
    } else if( field.attr("wccpf-type") === "radio" ) {             
        if( field.closest("ul").find("input[type=radio]").is(':checked') ) {
            field.closest("ul").next().hide();
        } else {
            field.closest("ul").next().show();
            this.isValid = false;                   
        }               
    } else if( field.attr("wccpf-type") === "checkbox" ) {          
        var values = field.closest("ul").find("input[type=checkbox]:checked").map(function() {
            return this.value;
        }).get();
        if( values.length === 0 ) {
            field.closest("ul").next().show();
            this.isValid = false;
        } else {                        
            field.closest("ul").next().hide();
        }           
    } else if( field.attr("wccpf-type") === "file" ) {              
        if( field.val() == "" ) {
            field.next().show();
            this.isValid = false;
        } else {
            field.next().hide();
        }                                   
    }
}