Capture network error on async component Vue 2

I'm spending quite a lot of time just to understand how I can handle network errors in the Async component in Vue v2.

so I want to display an error message if something wrong happens during API call.

here's the example of the code: codesandbox

Vue:

<child-component
  v-if="showChild"
  message="I am the child component."
  loadingMessage="Looking for child component..."
  errorMessage="Child component not available."
></child-component>

mounted() {
    var that = this;
    fetch("https://api.github.com/users/dasdasdasdasdasdas") //wrong url dasd... just for testing
      .then(function (res) {
        return res.json();
      })
      .then(function (data) {
        console.log(data);
        that.showChild = true;
      })
      .catch(function (err) {
        console.log(err);
      });
},

an async component's error state is only enabled if there is a rejection returned by the import promise, i.e. only when this line fails:

import(/* webpackChunkName: "childComp" */ "@/components/ChildComponent.vue")

This import is not tied to your fetch call in any way. If you want to display an error message if your fetch fails, you can just use a boolean flag variable that controls a different v-if error element/component:

<child-component
      v-if="showChild"
      message="I am the child component."
      loadingMessage="Looking for child component..."
      errorMessage="Child component not available."
></child-component>
<div v-if="showError">Error message here</div>

data() {
  return {
    showChild: false,
    showError: false
  };
},

mounted() {
  var that = this;
  fetch("https://api.github.com/users/dasdasdasdasdasdas")
    .then(function (res) {
      return res.json();
    })
    .then(function (data) {
      console.log(data);
      that.showChild = true;
    })
    .catch(function (err) {
      console.log(err);
      that.showError = true;
    });
},

By the way, your example URL https://api.github.com/users/dasdasdasdasdasdas does not return a network error. 4xx and 5xx error codes can actually be valid responses. If you want to enter your catch block you need to use a completely fake hostname, e.g. fake.url.com/api, and/or specifically handle 404s in your then() block:

.then(function (res) {
  if (res.ok) {
    return res.json();
  } else {
    throw new Error('to the catch block');
  }
})