文件上传前预览

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>

<body>
    <div id="preview"></div>
    <input type="file" />
    <img src="" alt="">
    <script type="text/javascript">

    $("input").onchange = function(e) {
        var file = e.target.files;

        var reader = new FileReader();
        reader.onload = function(evt) {
            $('img').src = evt.target.result;
        }
        reader.readAsDataURL(file[0]);
    }

    function $(id) {
        return document.getElementsByTagName(id)[0];
    }
    </script>
</body>

</html>