在JavaScript中获取下拉框的价值

Question

I'm trying to get the value of a drop down box and keep getting the error Uncaught TypeError: Cannot read property 'selectedIndex' of null.

Heres my code:

JS

expirationYearSelectedIndex = document.getElementById("selYear").selectedIndex;
    console.log("expirationYearSelectedIndex = " + expirationYearSelectedIndex);

    expirationYear = document.getElementById("selYear").options[expirationYearSelectedIndex].value;
    console.log("expirationYear = " + expirationYear);

HTML

<form action="">
    <select id="SelYear" name="selYear">
        <option selected value=""> Select a Major....</option>
        <option value="Comptuer Information Systems"> CIS</option>
        <option value="Math"> M</option>
        <option value="CS"> CS</option>
        <option value="History"> H</option>
    </select>
</form>

Answer 1

<select id="SelYear" name="selYear"> ID名称为<select id="SelYear" name="selYear">

document.getElementById("SelYear");