import TreeViewItemValue from './TreeViewItemValue.vue';
import TreeViewItem from './TreeViewItem.vue';
import TreeView from './TreeView.vue';

module.exports = function install(Vue){
    Vue.component("tree-view-item-value", TreeViewItemValue);
    Vue.component("tree-view-item", TreeViewItem);
    Vue.component("tree-view", TreeView);  
};

import TreeViewItemValue from './TreeViewItemValue.vue';
import TreeViewItem from './TreeViewItem.vue';
import TreeView from './TreeView.vue';

export default function install(Vue){
    Vue.component("tree-view-item-value", TreeViewItemValue);
    Vue.component("tree-view-item", TreeViewItem);
    Vue.component("tree-view", TreeView);  
};

export { TreeView }

Update index.js