Communicating Between Recursive Components in Vue.js

Imagine that you want to check when a NodeTree from the previous article gets clicked-on, and communicate it to the outer component (the one who’s using the Tree component).

In Vue.js, upwards communication happens using custom events. But we’ve learned from Vue.js 1.x or Angular 1.x that having events across multiple levels is a bad practice and makes for a codebase that’s hard to reason about. That’s why, starting with Vue.js 2, custom events are only allowed on one child-to-parent level.

Knowing that, imagine a Tree like the following:

+ Folder 1
  + Folder 2
    + Folder 3
      + Folder 4
        + Folder 5

If we’d like to communicate a click event from Folder 5 using custom events, we’d have to emit an event up 5 times.

We can say a tree has infinite levels of depth, since we don’t know beforehand how many it has. For that reason, using custom events would make it unfeasible, inefficient and complex.

Remember that we can use functions as props in Vue.js. It’s not a common pattern because, unlike with React, Vue.js has custom events for child-to-parent communication. But for cases like this one it really comes in handy.

Unlike emitting events at each level, this is much simpler and performant since we only need to pass down the same reference to the function.

Let’s add a handleClick property to the NodeTree.vue component, and use it for the click event:

<template>
  <li class="node-tree">
    <span class="label" @click="handleClick(node)">{{ node.label }}</span>

    <ul v-if="node.children && node.children.length">
      <node
        v-for="child in node.children"
        :node="child"
        :handle-click="handleClick">
      </node>
    </ul>
  </li>
</template>

<script>
export default {
  name: "node",
  props: {
    node: Object,
    handleClick: Function
  }
};
</script>

Notice how we set its type as Function and we use it on the the label @click event and pass it down again to the children nodes :handle-click="handleClick".

From the Tree.vue component, we still need to pass that function down to the root node.

Additionally, we must provide a way to handle the click from the outside (the component who uses Tree), and for this we can indeed use a custom event since it’s just one level up:

<template>
  <div class="tree">
    <ul class="tree-list">
      <node-tree :node="treeData" :handle-click="handleClick"></node-tree>
    </ul>
  </div>
</template>

<script>
import NodeTree from "./NodeTree";

export default {
  props: {
    treeData: Object
  },
  methods: {
    handleClick (node) {
      this.$emit('node-click', node);
    }
  }
  components: {
    NodeTree
  }
};
</script>

As you can see, we added a handleClick method local to the Tree component, which is passed to the node-tree. This method emits an event with the node of data by using the $emit Vue instance method.

With that, we can handle the click from App.vue or any other external component, while keeping a nice API syntax by using @node-click on the template:

<template>
  <div>
    <tree :tree-data="tree" @node-click="logClick"></tree>
  </div>
</template>

<script>
import Tree from "./Tree";

export default {
  data: () => ({
    // ...
  }),
  methods: {
    logClick(node) {
      console.log("Clicked: ", node);
    }
  },
  components: {
    Tree
  }
};
</script>

As you just saw, to fix the “infinite levels” communication problem, we’ve made use of a pattern that’s very common in the React community: passing a function down as a property. This allowed us to handle a click action easily on a recursive component while keeping it performant and easy to reason about.

Stay cool 🦄