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lifecycle-onmounted

Lifecycle: onMounted

Last reviewed May 28, 2026 Content v20260528
Track mode
client_vue
Means
In-browser Vue TS
Reading
~1 min
Level
intermediate

This lesson

This lesson teaches Lifecycle: onMounted: the concepts, APIs, and habits you need before advancing in Vue.

Without Lifecycle: onMounted, you will struggle to read or extend Vue codebases and playground exercises.

You will apply Lifecycle: onMounted in contexts like: Greenfield SPAs, dashboards, design systems, and full-stack apps that pair Vue with PHP or Node APIs.

Write TypeScript, click Run—Vue 3 loads from CDN with the template compiler, mountApp shows UI in #app, and printOutput feeds the terminal.

When you can explain the previous lesson's ideas without copying starter code.

Lifecycle hooks let you run code at specific moments in a component’s life. In Composition API, import hooks from Vue and call them inside setup().

Common hooks

  • onBeforeMount / onMounted — before/after DOM is created
  • onBeforeUpdate / onUpdated — around re-renders
  • onBeforeUnmount / onUnmounted — teardown (timers, subscriptions)

Typical onMounted uses

  • Fetch data when the component appears
  • Initialize third-party libraries that need DOM nodes
  • Focus an input or measure layout

Cleanup

Pair side effects with onUnmounted—clear intervals, abort fetches, remove listeners. Same discipline as React useEffect cleanup.

Self-check

  1. When does onMounted run relative to the first paint?
  2. Where should you clear a setInterval?

Challenge

Document title

  1. Type in the input and watch document.title update (check the browser tab in the iframe).
  2. Confirm onUnmounted resets the title when you re-run with different code.

Done when: document.title matches the input while the component is mounted.

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

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Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

  • Fetch in onMounted?
  • SSR caveat?

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