效果如上图↑

// 折叠所有节点
foldAllNodes(type) {
    let treeNodes = this.$refs[type + '_tree'].store._getAllNodes()
    for (var i = 0; i < treeNodes.length; i++) {
        treeNodes[i].expanded = false;
    }
},
// 展开所有节点
expandAllNodes(type) {
    let treeNodes = this.$refs[type + '_tree'].store._getAllNodes()
    for (var i = 0; i < treeNodes.length; i++) {
        treeNodes[i].expanded = true;
    }
},

然后务必在

 <el-tree

...
node-key="id" //加入这个属性，否者代码无法生效

...
</el-tree>