h function for creating vnodes in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the h function in Vue?

The h function creates virtual nodes (vnodes) that represent elements or components in Vue's virtual DOM. It helps Vue know what to render on the screen.

beginner

How do you create a simple div element with text using the h function?

Use h('div', 'Hello'). This creates a virtual div node with the text 'Hello' inside.

intermediate

What are the three main arguments of the h function?

1. Tag or component name (string or component object)
2. Props or attributes (object, optional)
3. Children (string, array, or vnode, optional)

intermediate

Can the h function create components as well as HTML elements?

Yes, you can pass a component object as the first argument to h to create a vnode for that component.

beginner

Why is the h function important in Vue's rendering process?

It builds a lightweight virtual tree of nodes that Vue compares to the real DOM. This makes updates efficient and fast.

What does the h function return in Vue?

A A string of HTML code

B A real DOM element

C A virtual node (vnode) representing an element or component

D A CSS style object

The h function returns a vnode, which is a lightweight description of what to render.

Which argument is NOT valid for the h function?

A Tag name as a string

B Props object

C Children as a string or array

D A CSS selector string

The h function does not accept CSS selector strings; it expects tag names or component objects.

How do you pass attributes like id or class using h?

A Inside the children argument

B As the second argument, an object with keys like <code>id</code> or <code>class</code>

C By adding them to the tag string

D You cannot pass attributes with <code>h</code>

Attributes are passed as an object in the second argument to h.

Can the children argument of h be an array?

A Yes, to create multiple child nodes

B No, it must be a string

C No, it must be a single vnode

D Only if the first argument is a component

Children can be an array of vnodes or strings to create multiple children.

Which Vue feature uses the h function under the hood?

A The template compiler

B Vue Router

C Vuex store

D Vue Devtools

Vue's template compiler converts templates into render functions that use h to create vnodes.

Explain how the h function helps Vue render elements and components.

Think about how Vue builds a virtual tree before showing things on the screen.

Describe the three main arguments of the h function and what each one does.

Remember the order: what to create, its properties, and its content.

Practice

1. What is the primary purpose of the h function in Vue?

easy

A. To define CSS styles for components

B. To directly manipulate the DOM elements

C. To fetch data from an API

D. To create virtual nodes representing elements or components

Solution

1. Step 1: Understand the role of h in Vue

   The h function is used to create virtual nodes (vnodes) which represent elements or components in Vue's virtual DOM.

2. Step 2: Differentiate from other functions

   It does not manipulate the real DOM directly, fetch data, or style components. Its main role is to build the virtual DOM structure.

3. Final Answer:

   To create virtual nodes representing elements or components -> Option D

4. Quick Check:

   h function = create virtual nodes [OK]

Hint: Remember: h builds virtual nodes, not real DOM [OK]

Common Mistakes:

• Confusing h with direct DOM manipulation
• Thinking h fetches data
• Assuming h styles components

2. Which of the following is the correct syntax to create a div element with the h function and a class of "container"?

easy

A. h('div', ['class', 'container'])

B. h('div', { class: 'container' })

C. h('div', 'class', 'container')

D. h('div', { 'className': 'container' })

Solution

1. Step 1: Recall the h function signature

   The h function takes the tag name as the first argument and an object of props as the second argument.

2. Step 2: Identify correct props syntax

   Vue uses class (not className) as the key for CSS classes in the props object.

3. Final Answer:

   h('div', { class: 'container' }) -> Option B

4. Quick Check:

   h(tag, props) with class key = A [OK]

Hint: Props are an object; use { class: 'name' } [OK]

Common Mistakes:

• Using 'className' instead of 'class'
• Passing class as separate string arguments
• Using array instead of object for props

3. What will be the rendered output of this Vue render function snippet?

return h('ul', null, [
  h('li', null, 'Apple'),
  h('li', null, 'Banana'),
  h('li', null, 'Cherry')
])

medium

A. AppleBananaCherry

B. AppleBananaCherry

C. AppleBananaCherry

D. Error: Children must be strings

Solution

1. Step 1: Understand the structure created by h

   The outer h creates a ul element with an array of li children nodes.

2. Step 2: Recognize the children rendering

   Each li vnode contains a string child representing the fruit name, so the final HTML is a list with three li items.

3. Final Answer:

   <ul><li>Apple</li><li>Banana</li><li>Cherry</li></ul> -> Option A

4. Quick Check:

   h with array children = nested elements [OK]

Hint: Array children create nested elements inside parent [OK]

Common Mistakes:

• Thinking children array concatenates text
• Expecting error for array children
• Missing the parent <ul> element

4. Identify the error in this usage of the h function:

return h('button', { onclick: () => alert('Clicked!') }, 'Click me')

medium

A. The event name should be lowercase: 'onclick'

B. The event handler should be passed as 'onClick' but as a string

C. The event handler should be passed as 'onClick' with capital C

D. The event handler should be passed as 'onClick' but inside an array

Solution

1. Step 1: Recall Vue event listener naming in h

   Vue expects event listeners in props to be prefixed with 'on' and camelCase event name, e.g., 'onClick'.

2. Step 2: Identify the incorrect casing

   The code uses 'onclick' with lowercase 'c', which Vue does not recognize as an event listener.

3. Final Answer:

   The event handler should be passed as 'onClick' with capital C -> Option C

4. Quick Check:

   Event props use camelCase event names [OK]

Hint: Use camelCase event names like 'onClick' in props [OK]

Common Mistakes:

• Using lowercase event names
• Passing event handler as string
• Wrapping handler in array unnecessarily

5. How would you use the h function to create a ul list from this array of fruits, but only include fruits with names longer than 5 characters?

const fruits = ['Apple', 'Banana', 'Fig', 'Cherry']

hard

A. h('ul', null, fruits.filter(f => f.length > 5).map(f => h('li', null, f)))

B. h('ul', null, fruits.map(f => f.length > 5 ? h('li', null, f) : f))

C. h('ul', null, fruits.map(f => h('li', null, f.length > 5)))

D. h('ul', null, fruits.filter(f => f.length > 5))

Solution

1. Step 1: Filter fruits by length

   Use filter to keep only fruits with names longer than 5 characters.

2. Step 2: Map filtered fruits to li vnodes

   Use map to convert each filtered fruit into an li vnode with the fruit name as child.

3. Step 3: Pass the array of li vnodes as children to ul

   This creates a ul with only the desired fruits as list items.

4. Final Answer:

   h('ul', null, fruits.filter(f => f.length > 5).map(f => h('li', null, f))) -> Option A

5. Quick Check:

   Filter then map to
6. nodes = A [OK]

Hint: Filter first, then map to <li> nodes inside h [OK]

Common Mistakes:

• Mapping without filtering includes unwanted items
• Passing booleans or nulls as children
• Passing raw strings array without wrapping in <li>