Higher-order components concept in Vue

Introduction

A higher-order component (HOC) is a way to reuse component logic by wrapping one component inside another. It helps keep your code clean and organized.

When you want to add the same behavior or data to many components without repeating code.

When you need to share common logic like loading data or handling errors across components.

When you want to enhance a component with extra features like tracking user actions.

When you want to separate concerns by keeping UI and logic apart.

When you want to wrap a component to add styling or layout without changing its code.

Syntax

Vue

const EnhancedComponent = (BaseComponent) => {
  return {
    components: { BaseComponent },
    setup(props, { attrs, slots }) {
      // shared logic here
      return () => h(BaseComponent, { ...attrs }, slots)
    }
  }
}

The HOC is a function that takes a component and returns a new component.

Use h() to render the wrapped component inside the new one.

Examples

This HOC logs a message every time the wrapped component renders.

Vue

const withLogger = (BaseComponent) => {
  return {
    setup(props, { attrs, slots }) {
      return () => {
        console.log('Component rendered')
        return h(BaseComponent, { ...attrs }, slots)
      }
    }
  }
}

This HOC shows a loading message before displaying the wrapped component.

Vue

const withLoading = (BaseComponent) => {
  return {
    data() {
      return { loading: true }
    },
    mounted() {
      setTimeout(() => { this.loading = false }, 2000)
    },
    render() {
      if (this.loading) return h('div', 'Loading...')
      return h(BaseComponent, this.$attrs, this.$slots)
    }
  }
}

Sample Program

This example creates a higher-order component withBorder that adds a blue border around any component it wraps. We wrap SimpleMessage to get BorderedMessage which shows the message inside a bordered box.

Vue

<script setup>
import { h } from 'vue'

const withBorder = (BaseComponent) => {
  return {
    setup(props, { attrs, slots }) {
      return () => h('div', { style: { border: '2px solid blue', padding: '1rem' } }, [
        h(BaseComponent, { ...attrs }, slots)
      ])
    }
  }
}

const SimpleMessage = {
  props: ['msg'],
  template: `<p>{{ msg }}</p>`
}

const BorderedMessage = withBorder(SimpleMessage)
</script>

<template>
  <BorderedMessage msg="Hello from HOC!" />
</template>

Important Notes

HOCs help keep your code DRY (Don't Repeat Yourself) by sharing logic.

Remember to pass props and slots correctly to the wrapped component.

Vue 3 uses h() function to create elements inside setup or render functions.

Summary

Higher-order components wrap other components to add shared logic or styling.

They are functions that return new components wrapping the original ones.

Use HOCs to keep your Vue app clean and reusable.

Practice

1. What is the main purpose of a higher-order component (HOC) in Vue?

easy

A. To create a new Vue instance from scratch

B. To replace the Vue router with a new navigation system

C. To wrap a component and add shared logic or styling

D. To directly manipulate the DOM elements inside a component

Solution

1. Step 1: Understand what HOCs do

   Higher-order components wrap existing components to add extra features without changing the original component.

2. Step 2: Identify the correct purpose

   Among the options, only wrapping a component to add shared logic or styling matches the HOC concept.

3. Final Answer:

   To wrap a component and add shared logic or styling -> Option C

4. Quick Check:

   HOC purpose = wrap and add logic [OK]

Hint: HOCs wrap components to add features without changing them [OK]

Common Mistakes:

• Confusing HOCs with routing or Vue instance creation
• Thinking HOCs manipulate DOM directly
• Believing HOCs replace components instead of wrapping

2. Which of the following is the correct syntax to create a higher-order component in Vue 3 using the Composition API?

easy

A. const withLogger = (WrappedComponent) => { return { setup() { return () => h(WrappedComponent) } } }

B. const withLogger = (WrappedComponent) => { return WrappedComponent.setup() }

C. const withLogger = (WrappedComponent) => { return new Vue({ render: () => h(WrappedComponent) }) }

D. const withLogger = (WrappedComponent) => { return () => WrappedComponent }

Solution

1. Step 1: Review HOC syntax in Vue 3

   A HOC is a function that returns a new component wrapping the original. Using setup and render function with h() is correct.

2. Step 2: Check each option

   const withLogger = (WrappedComponent) => { return { setup() { return () => h(WrappedComponent) } } } returns a component object with setup returning a render function that renders WrappedComponent, which is correct.

3. Final Answer:

   const withLogger = (WrappedComponent) => { return { setup() { return () => h(WrappedComponent) } } } -> Option A

4. Quick Check:

   HOC returns component with setup and render [OK]

Hint: HOCs return a new component wrapping the original with setup [OK]

Common Mistakes:

• Calling setup() directly instead of returning a component
• Using new Vue() in Vue 3 Composition API
• Returning the component function without wrapping

3. Given this HOC code in Vue 3:

const withBorder = (WrappedComponent) => {
  return {
    setup() {
      return () => h('div', { style: { border: '2px solid red' } }, [h(WrappedComponent)])
    }
  }
}

const Base = { template: '
Hello
' }
const Enhanced = withBorder(Base)

What will <Enhanced /> render?

medium

A. Hello

B. Hello

C. Hello

D. Error: h() cannot wrap components

Solution

1. Step 1: Understand the HOC rendering

   The HOC wraps the WrappedComponent inside a div with a red border style.

2. Step 2: Predict the rendered output

   Rendering <Enhanced /> produces a div with red border containing the Base component's paragraph.

3. Final Answer:

   <div style="border: 2px solid red"><p>Hello</p></div> -> Option B

4. Quick Check:

   HOC adds border div around component [OK]

Hint: HOCs wrap output in extra elements or logic [OK]

Common Mistakes:

• Ignoring the wrapping div and expecting only <p>
• Thinking h() cannot wrap components
• Assuming no styles are applied

4. Identify the error in this Vue 3 HOC code:

const withTitle = (WrappedComponent) => {
  return {
    setup() {
      return () => h(WrappedComponent)
    }
  }
}

const Base = { template: '
Hi
' }
const Enhanced = withTitle(Base)

// Usage: <Enhanced title="Welcome" />

What is the problem?

medium

A. The HOC does not pass props to WrappedComponent

B. The HOC should use a class component instead

C. The Base component must be a function, not an object

D. The setup function cannot return a render function

Solution

1. Step 1: Check prop forwarding in HOC

   The HOC returns WrappedComponent but does not forward any props it receives.

2. Step 2: Understand consequence

   When using <Enhanced title="Welcome" />, the title prop is lost because it's not passed down.

3. Final Answer:

   The HOC does not pass props to WrappedComponent -> Option A

4. Quick Check:

   Props must be forwarded in HOCs [OK]

Hint: Always forward props from HOC to wrapped component [OK]

Common Mistakes:

• Forgetting to pass props in render function
• Thinking setup can't return render function
• Believing Base must be a function component

5. You want to create a higher-order component withLoading that shows a loading message while data is loading, then renders the wrapped component with the loaded data. Which approach correctly combines HOC logic with Vue 3 Composition API?

hard

A. Create a mixin that adds loading state and use it inside the wrapped component

B. Modify the wrapped component's template directly to add loading message

C. Use a global variable to track loading and conditionally render wrapped component

D. Return a component with setup() that uses ref for loading state, fetches data, and renders loading or wrapped component with props

Solution

1. Step 1: Understand HOC with loading logic

   The HOC should manage loading state internally and decide what to render based on it.

2. Step 2: Identify correct Vue 3 pattern

   Using setup with ref for loading, fetching data, and returning a render function that shows loading or wrapped component with data is correct.

3. Final Answer:

   Return a component with setup() that uses ref for loading state, fetches data, and renders loading or wrapped component with props -> Option D

4. Quick Check:

   HOC manages state and renders conditionally [OK]

Hint: Use setup with state and conditional render in HOC [OK]

Common Mistakes:

• Changing wrapped component template directly
• Using global variables for state
• Confusing mixins with HOCs