javascript函数验证单选按钮给出奇怪的输出。

I'm developing a quiz that pulls data out of a mysql database, and displays the results as radio buttons. The radio buttons are populated based off of key=>value and generated via a simple forloop. This has been done many times, simple google searching and research will yield all the results needed to accomplish this. The issue that I'm having, or was having (before I decided to just do this with jquery) was when I submit the form it would execute the javascript function to validate whether a button has been selected, but when you select any option other than the first radio button you'd receive the same "make a selection" alert that you would if you had not selected any buttons. Selecting the first radio button would return true and execute the getCheckedValue function call. It seems as though, the script only recognizes that I have one input type and doesn't understand to iterate through the rest of the buttons. I've refactored this function a dozen times, and still have no idea why this doesn't work.

<?php
    foreach ($dataReturn as $j => $value){
       echo "<input type='radio' class='answer' id='radiobtn' name='radiobtn' value='".$j."'>" .$value." </input><br/>";
    }
?>

Above is the loop that generates the radio buttons (just for reference, $dataReturn is the return value of a shuffled associative array. (Which is working as intended) When the submit button is clicked, it calls the below javascript function.

    function isNull(){
    var isChecked = false;
    var radiobutton = document.getElementsByName('radiobtn');
    for (var i=0; i <= radiobutton.length; i++){
        if (radiobutton[i].checked){
            return true;
            var answer = radiobutton[i].value;
            getCheckedValue(answer);//using this just for testing selected value
        }else { 
        alert("Make a selection.");
        }
    return false;
    }
}

I just can't figure out why this doesn't work. As stated above, using jquery this works perfectly.

1. Your FOR loop: since JS uses zero-based arrays, you can't have <=, otherwise it will look for an index one higher than what you have. Use < instead;
2. I moved your validation for whether any fields were checked outside the loop to make management easier. It's cleaner this way than worrying about breakout out of loops in the middle of them.

Here:

function isNull() {
    var isChecked = false;
    var radiobutton = document.getElementsByName('radiobtn');
    for (var i=0; i < radiobutton.length; i++) {
        if (radiobutton[i].checked) {
            isChecked = true;
        }
    }

    if ( !isChecked ) {
        alert("Make a selection.");
        return false;
    }
}

I don't know how your form tag looks, but here is what you need to prevent the form from submitting if no radio fields are checked:

<form action="" method="post" onSubmit="return isNull();">

Try the code below. You do not want your alert to fire or return false until after the for loop is finished.

 function isNull(){
    var isChecked = false;
    var radiobutton = document.getElementsByName('radiobtn');
    for (var i=0; i <= radiobutton.length; i++){
        if (radiobutton[i].checked){
            var answer = radiobutton[i].value;
            getCheckedValue(answer);//using this just for testing selected value
            return true;
        }
    }
    alert("Make a selection.");
    return false;
}

Also, your php code gives all radio buttons the same id. That is bad; doing so violates w3c standards.