Fetch API in Vue components - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Fetch API used for in Vue components?

The Fetch API is used to get data from a server or send data to a server asynchronously inside Vue components.

beginner

How do you trigger a fetch request when a Vue component loads?

You use the onMounted lifecycle hook to run the fetch request when the component is first shown.

intermediate

Why should you use ref or reactive when fetching data in Vue?

Using ref or reactive makes the fetched data reactive, so the component updates automatically when data changes.

intermediate

What is a common way to handle errors when using Fetch API in Vue?

You catch errors with try...catch inside an async function or use .catch() on the fetch promise to handle problems gracefully.

beginner

How can you show a loading state while fetching data in a Vue component?

Create a reactive variable like isLoading set to true before fetch, then set it to false after data loads to show or hide a loading message.

Which Vue lifecycle hook is best for fetching data when a component appears?

A onUpdated

B onMounted

C onBeforeUnmount

D onCreated

The onMounted hook runs after the component is added to the page, perfect for fetching data.

What does the Fetch API return when you call fetch(url)?

A A promise that resolves to a Response object

B The data directly

C An error object

D A Vue reactive variable

Fetch returns a promise that resolves to a Response object, which you then process to get data.

How do you make fetched data reactive in Vue 3 Composition API?

A Use ref() or reactive() to store the data

B Store data in a normal variable

C Use computed() only

D Use watch() only

ref() or reactive() create reactive data containers that update the UI when changed.

Which method converts a fetch Response to JSON?

A response.data()

B response.text()

C response.json()

D response.parse()

response.json() parses the response body as JSON.

What is a good practice to handle fetch errors in Vue?

A Use alert() without catching errors

B Ignore errors

C Reload the page automatically

D Use try...catch inside async functions

Using try...catch helps catch and handle errors gracefully.

Explain how to fetch data in a Vue component using the Composition API.

Think about when the component loads and how to keep data reactive.

Describe how to show a loading message while fetching data in Vue.

Consider how to track and display loading status.

Practice

1. What is the main purpose of using the Fetch API inside a Vue component?

easy

A. To handle user input events like clicks

B. To style the component with CSS

C. To get data from the internet and use it in the component

D. To create new Vue components dynamically

Solution

1. Step 1: Understand Fetch API role

   The Fetch API is used to request data from a server or internet resource.

2. Step 2: Connect Fetch API to Vue component

   Inside Vue components, fetch is used to load data that the component will display or use.

3. Final Answer:

   To get data from the internet and use it in the component -> Option C

4. Quick Check:

   Fetch API = get data [OK]

Hint: Fetch API fetches data from internet for Vue components [OK]

Common Mistakes:

• Thinking Fetch API styles components
• Confusing fetch with event handling
• Using fetch to create components

2. Which is the correct way to use fetch inside a Vue 3 <script setup> component to load data on mount?

easy

A. Call fetch inside the onMounted() lifecycle hook

B. Call fetch inside a computed property

C. Use fetch directly in the template section

D. Use fetch inside a watchEffect without dependencies

Solution

1. Step 1: Identify lifecycle hook for data loading

   onMounted() runs code after the component appears, perfect for fetching data.

2. Step 2: Match fetch usage to lifecycle hook

   Calling fetch inside onMounted() ensures data loads once component is ready.

3. Final Answer:

   Call fetch inside the onMounted() lifecycle hook -> Option A

4. Quick Check:

   Fetch in onMounted = correct usage [OK]

Hint: Use onMounted() to fetch data after component loads [OK]

Common Mistakes:

• Trying to fetch data in template (not allowed)
• Using computed for async fetch (wrong usage)
• Using watchEffect without dependencies causing repeated fetch

3. What will be the output in the console when this Vue 3 <script setup> code runs?

import { ref, onMounted } from 'vue';
const data = ref(null);
const error = ref(null);
onMounted(async () => {
  try {
    const res = await fetch('https://jsonplaceholder.typicode.com/posts/1');
    data.value = await res.json();
    console.log(data.value.title);
  } catch (e) {
    error.value = 'Failed to load';
    console.log(error.value);
  }
});

medium

A. Logs undefined because data.value is never set

B. Logs 'Failed to load' always

C. Throws a syntax error because async is not allowed

D. Logs the title of the post with id 1 from the API

Solution

1. Step 1: Analyze fetch and await usage

   The code fetches a post from the API and waits for the JSON response.

2. Step 2: Check console.log output

   It logs the title property of the fetched post, so it prints the post's title.

3. Final Answer:

   Logs the title of the post with id 1 from the API -> Option D

4. Quick Check:

   Fetch success logs title [OK]

Hint: Await fetch then log data property for output [OK]

Common Mistakes:

• Assuming fetch always fails
• Thinking async is not allowed in onMounted
• Expecting undefined because of missing await

4. Identify the error in this Vue 3 <script setup> snippet that fetches data:

import { ref, onMounted } from 'vue';
const data = ref(null);
const error = ref(null);
onMounted(() => {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(json => data = json)
    .catch(() => error.value = 'Error fetching');
});

medium

A. Assigning json directly to data instead of data.value

B. Missing await keyword before fetch

C. Using onMounted instead of created lifecycle hook

D. Not returning the fetch promise

Solution

1. Step 1: Check how ref is updated

   Refs in Vue must be updated by setting their .value property, not the ref variable itself.

2. Step 2: Identify incorrect assignment

   The code assigns json directly to data, which breaks reactivity; it should be data.value = json.

3. Final Answer:

   Assigning json directly to data instead of data.value -> Option A

4. Quick Check:

   Update ref with .value [OK]

Hint: Update ref variables with .value, not direct assignment [OK]

Common Mistakes:

• Forgetting .value when setting ref
• Confusing async/await with then chaining
• Thinking onMounted is wrong lifecycle hook here

5. You want to fetch user data and show a loading message while waiting. Which Vue 3 <script setup> code correctly implements fetch with loading and error states?

hard

A. const loading = false;
const error = null;
const user = null;
onMounted(() => {
fetch('/user')
.then(res => res.json())
.then(data => user = data)
.catch(() => error = 'Failed');
loading = false;
});

B. const loading = ref(false);
const error = ref(null);
const user = ref(null);
onMounted(async () => {
loading.value = true;
try {
const res = await fetch('/user');
user.value = await res.json();
} catch {
error.value = 'Failed to load user';
} finally {
loading.value = false;
}
});

C. const loading = ref(true);
const error = ref('');
const user = ref({});
fetch('/user').then(res => res.json()).then(data => {
user.value = data;
loading.value = false;
}).catch(() => {
error.value = 'Error';
loading.value = false;
});

D. const loading = ref(false);
const error = ref(null);
const user = ref(null);
async function loadUser() {
loading.value = true;
const res = await fetch('/user');
user.value = await res.json();
loading.value = false;
}
onMounted(loadUser);

Solution

1. Step 1: Check reactive state setup

   const loading = ref(false); const error = ref(null); const user = ref(null); onMounted(async () => { loading.value = true; try { const res = await fetch('/user'); user.value = await res.json(); } catch { error.value = 'Failed to load user'; } finally { loading.value = false; } }); uses ref() for loading, error, and user, enabling reactivity and UI updates.

2. Step 2: Verify fetch with loading and error handling

   It sets loading true before fetch, uses try/catch for errors, and sets loading false in finally block.

3. Final Answer:

   Uses refs with onMounted, try/catch/finally for loading and error states -> Option B

4. Quick Check:

   Loading true before fetch, false after [OK]

Hint: Use ref and try/catch with finally to manage loading state [OK]

Common Mistakes:

• Not using ref for reactive state
• Setting loading false too early
• Missing error handling in async fetch