<body>

    <div id="i1">

        学习是我快乐？

        <a>晚饭吃什么</a>

    </div>

</body>

var obj = document.getElementById("i1")

obj

<div id=​"i1">​…​</div>​

obj.innerText

"学习是我快乐？ 晚饭吃什么"

// innerText 不能获取对象中的<a>、<span>等标签元素，只能获取文本内容

obj.innerText = '<a herf="https://www.google.com">谷歌</a>'

"<a herf="https://www.google.com">谷歌</a>"

var obj = document.getElementById("i1")

obj.innerHTML

"

        学习是我快乐？

        <a>晚饭吃什么</a>

    "

obj.innerHTML='<a href="https://www.google.com">谷歌</a>'

"<a href="https://www.google.com">谷歌</a>"

<body>

    <div id="i1">

        <input type="text" id="i2" />

    </div>

</body>

var obj = document.getElementById("i2")

undefined

obj.value = "君的名字"

"君的名字"

<body>

    <div id="i1">

        <textarea id="i4">sssssss</textarea>

    </div>

</body>

<body>

    <div style="width:600px;margin:0 auto;">

        <input id="i1" onfocus="Focus();" onblur="Blur();" type="text" value="请输入关键字" />

    </div>

    <script>

        function Focus(){

            var tag = document.getElementById("i1");

            var val = tag.value;

            if(val === "请输入关键字"){

                tag.value = "";

            }

    }

    function Blur(){

        var tag = document.getElementById("i1");

        var val = tag.value;

        if(val.length === 0){

            tag.value = "请输入关键字";

        }

    }

    </script>

</body>