Question

How do, when click a checkbox, in var check=''; will be added – okay,
and if click again,okay will be removed.

How it make?

Answer 1

var check = "";
$("#yourcheckbox").change(function() {
    if($(this).is(":checked")) {
        check = "okay";
    }
    else {
        check = "";
    }
});

This does exactly what you ask, but the solution posted by @Rikudo is better if you can live without check having to contain “okay”.