如果未选中复选框,如何使表单不提交

function checkCheckbox() {
  var checkbox = document.getElementById("myCheckbox");
  if (!checkbox.checked) {
    alert("请先勾选复选框");
    return false;
  }
  return true;
}

<form onsubmit="return checkCheckbox()">
 <label>
   <input type="checkbox" id="myCheckbox" name="myCheckbox" value="1">
    我已阅读并同意条款
  </label>
 <button type="submit">提交</button>
</form>