参考效果：
http://www.17sucai.com/previe…

引入cropper使用

<link  href="/path/to/cropper.css" rel="stylesheet">
<script src="/path/to/cropper.js"></script>

HTML结构

<li class="ui-border-bottom">
    <a href="javascripts:void(0);">头像
        <span class="pull-right user-header">
            <img class="rounded" id="avatar" src="../images/user.jpg" alt="avatar">
            <input type="file" class="sr-only" id="input" name="image" accept="image/*">
        </span>
    </a>
</li>    

<!–头像截图弹窗–>

<div class="common-layer" id="modal">
    <div class="layer-content">
        <div class="layer-title">
            <span class="cancel-btn" id="cancle">取消</span>
            <h4>截图头像</h4>
        </div>
        <div class="layer-area">
            <div class="img-container">
                <img id="image" src="https://avatars0.githubusercontent.com/u/3456749">
            </div>
            <a class="ui-btn m-t-15" id="crop" href="javascripts:void(0);">保存</a>
        </div>
    </div>
</div>

js引用：
具体cropper.js 使用可查看官网http://fengyuanchen.github.io…

$(function() {
    //修改头像 参加文件https://blog.csdn.net/weixin_38023551/article/details/78792400
    var avatar = document.getElementById('avatar');
    var image = document.getElementById('image');
    var input = document.getElementById('input');
    var $modal = $('#modal');
    var cropper;
    //点击图片
    input.addEventListener('change', function (e) {
        var files = e.target.files;
        var done = function (url) {
            input.value = '';
            image.src = url;
            $modal.show(function() {
                //初始化
                cropper = new Cropper(image, {
                    aspectRatio: 1,
                    viewMode:1,
                });
            });

        };
        var reader;
        var file;
        var url;

        if (files && files.length > 0) {
            file = files[0];
            if (URL) {
                done(URL.createObjectURL(file));
            } else if (FileReader) {
                reader = new FileReader();
                reader.onload = function (e) {
                    done(reader.result);
                };
                reader.readAsDataURL(file);
            }
        }
    });
    //关闭弹窗
    document.getElementById('cancle').addEventListener('click', function () {
        $modal.hide(function() {
            cropper.destroy();
            cropper = null;
        });
    });
    //保存截图
    document.getElementById('crop').addEventListener('click', function () {
        var initialAvatarURL;
        var canvas;
        $modal.hide(function() {
            cropper.destroy();
            cropper = null;
        });

        if (cropper) {
            canvas = cropper.getCroppedCanvas({
                width: 120,
                height: 120,
            });
            initialAvatarURL = avatar.src;
            avatar.src = canvas.toDataURL('image/jpeg');
            //保存数据
            canvas.toBlob(function (blob) {
                var formData = new FormData();
                formData.append('avatar', blob);
                $.ajax('https://jsonplaceholder.typicode.com/posts', {
                    method: 'POST',
                    data: formData,
                    processData: false,
                    contentType: false,
                    success: function () {
                        console.log('Upload success');
                    },
                    error: function () {
                        avatar.src = initialAvatarURL;
                        console.log('Upload error');
                    }
                });
            });
        }
    });
})