求助，写成js函数

[slysmart]

<select onchange="selB.options[selectedIndex].selected=true">
<option>testA1</option>
<option>testA2</option>
<option>testA3</option>
<option>testA4</option>
<option>testA5</option>
</select>
    
<select id="selB">
<option>testB1</option>
<option>testB2</option>
<option>testB3</option>
<option>testB4</option>
<option>testB5</option>
</select>

各位，如何将selB.options[selectedIndex].selected=true写成js函数

[葉幺]

代码如下：
<select id="selA">
<option>testA1</option>
<option>testA2</option>
<option>testA3</option>
<option>testA4</option>
<option>testA5</option>
</select>
    
<select id="selB">
<option>testB1</option>
<option>testB2</option>
<option>testB3</option>
<option>testB4</option>
<option>testB5</option>
</select>
<script>
var selA = document.getElementById("selA");
var selB = document.getElementById("selB");
selA.onchange = function(){
// console.log(this.options.selectedIndex) // 获取点击后的下标
selB.options[this.options.selectedIndex].selected=true
}
</script>