radio value 가져오기

function go() {
    var a = document.getElementsByName("tab1");
    var b = document.getElementsByName("tab1-value");
    for(var i=0; i<a.length; i++){
        if(a[i].checked === true){
            b.value = a[i].value;
        }
    }
    document.form1.submit();
}

<input id="domestic-tap" type="radio" name="tab1" checked="checked" value="국내선">
<input type="text" name="tab1-value" id="tab1-value" value="">

.value = 값

.checked = 논리값 ( 클릭되어있으면 true 반환 안되어있으면 false 반환)