const tools = document.getElementsByName("tool");
for (let i = 0; i <tools.length ; i++) {
if(tools[i].checked) console.log(tools[i].value);
}
<input type="radio" name="tool" checked="checked" value="pencil" id="pencil">画笔
<br/>
<input type="radio" name="tool" value="eraser" id="eraser">橡皮