Just copy this function.

function InitCheckAll(checkAllSelector, checkboxSelector) {
    $(checkAllSelector).change(function() {
        var e = this;
       
        $(checkboxSelector).each(function() {
            this.checked = e.checked;
        });
    });

    $(checkboxSelector).change(function() {
        if (this.checked == true) {
            var isCheckAll = true;
            $(checkboxSelector).each(function() {
                if(!this.checked) {
                    return isCheckAll = false;
                }
            });
            $(checkAllSelector)[0].checked = isCheckAll;
        } else {
            if ($(checkAllSelector)[0].checked == true) {
                $(checkAllSelector)[0].checked = false;
            }
        }
    });
}

Then you can declare select/deselect all checkbox just a line code. Notice that the paramaters should be selector string. For example:

InitCheckAll('#yourMasterCheckBoxID', '.yourChildCheckBoxClass')