测试输入长度

Question

如果输入字母的长度等于0但不起作用，我希望弹出警报

<div class="rame">
    <input type=text placeholder="Name" style="position: relative; right:-750px" id="inp1">
    <input type=text placeholder="Name2" style="position: relative; right:-250px" id="inp2">
    <button onclick="check()">submit</button>
</div>

<script>
    function check() {
        var a = document.getElementById("inp1");
        var b = document.getElementById("inp2");
        if (a.length = 0) {
            alert("Hello World")
        }
    }
</script>

Answer 1

您应该致电if(a.value.length==0)。请注意=和==之间的区别。

Answer 2

1. 通过getElementById，您将获得一个ELEMENT
2. 在纯JavaScript中，长度是指字符串
3. 如果要比较，则必须使用double =

我认为您尝试过此操作

<div class="rame">
        <input type=text placeholder="Name" style="position: relative; right:-750px" id="inp1">
        <input type=text placeholder="Name2" style="position: relative; right:-250px" id="inp2">
        <button onclick="check()">submit</button>
    </div>

    <script>
        function check() {
            var a = document.getElementById("inp1");
            var b = document.getElementById("inp2");
            if (a.value.length == 0) {
                alert("Hello World")
            }
        }
    </script>