JavaScript 写一个方法，能够实现：获得某元素节点指定标签名的子元素节点，并且写出测试用例

var oDiv1 = document.getElementById("div1");
console.log(getChildren(oDiv1, 'div'));

function getChildren (ele, tag) {
  var children = ele.childNodes;
  if (typeof tag === 'string') {
    tag = tag.toUpperCase();
    var a = [];
    for (let i = 0; i < children.length; i++) {
      const node = children[i];
      if (node.nodeType === 1 && node.nodeName === tag) {
        a.push(node);
      }
    }
    return a;
  } else if (typeof tag === 'undefined') {
    var a = [];
    for (let j = 0; j < children.length; j++) {
      const nodej = children[i];
      if (nodej.nodeType === 1) {
        a.push(nodej);
      }
    }
    return a;
  } else {
    throw new Error("Tag参数类型错误!")
  }
}