7. What is the role of directives in Vue.js?

Basic

7. What is the role of directives in Vue.js?

Overview

Directives in Vue.js are special tokens in the markup that tell the library to do something to a DOM element. They are a powerful way to manipulate the DOM directly and are an essential part of Vue.js, providing a simple and declarative way to extend the functionality of HTML elements. Understanding directives is crucial for any developer working with Vue.js as they play a significant role in creating reactive and interactive web applications.

Key Concepts

  1. Reactivity: Directives are a key player in Vue's reactive system, updating the DOM when your state changes.
  2. Binding: They are used for binding data to the DOM, listening to events, and more, making the interaction between Vue instances and the DOM seamless.
  3. Custom Directives: Beyond the standard directives, Vue allows the creation of custom directives for reusable and complex DOM manipulations.

Common Interview Questions

Basic Level

  1. What is a directive in Vue.js and can you name a few common ones?
  2. How do you bind an element's title attribute to a Vue.js data property?

Intermediate Level

  1. Explain the difference between v-show and v-if directives.

Advanced Level

  1. How would you create a custom directive in Vue.js that auto-focuses an input field when a page loads?

Detailed Answers

1. What is a directive in Vue.js and can you name a few common ones?

Answer: A directive in Vue.js is a special token in the markup that tells the library to do something to a DOM element. Some common directives in Vue.js include v-model, for two-way data binding on form elements; v-if, for conditional rendering of elements; v-for, for rendering a list of items; and v-on, for attaching event listeners.

Key Points:
- Directives are prefixed with v- to indicate that they are special attributes provided by Vue.
- They provide a way to extend HTML with Vue.js-specific functionality.
- Directives are reactive, meaning they automatically update the DOM when the underlying data changes.

Example:

// Unfortunately, Vue.js code cannot be accurately represented with C# syntax.
// Please refer to JavaScript examples for Vue.js directives.

2. How do you bind an element's title attribute to a Vue.js data property?

Answer: You can bind an element's title attribute to a Vue.js data property using the v-bind directive or its shorthand :. This directive dynamically binds one or more attributes, or a component prop to an expression.

Key Points:
- v-bind is used to reactively update HTML attributes based on component data.
- The shorthand syntax : simplifies the directive usage for readability.
- This binding is reactive, so if the data property changes, the bound attribute will automatically update.

Example:

// Vue.js example with JavaScript
<template>
  <div v-bind:title="message">
    Hover over me!
  </div>
</template>

<script>
export default {
  data() {
    return {
      message: 'Hello Vue!'
    }
  }
}
</script>

3. Explain the difference between v-show and v-if directives.

Answer: Both v-show and v-if directives conditionally render elements, but they do so differently. v-if is "real" conditional rendering because it ensures that the element and its children are not rendered if the condition is false. On the other hand, v-show simply toggles the display CSS property of the element, meaning the element always remains in the DOM but is not visible when the condition is false.

Key Points:
- v-if is more performant for conditional rendering that won't change often since it removes and recreates elements.
- v-show is more performant for frequent toggles since the cost of keeping the element in the DOM is lower than creating and destroying it.
- Choosing between v-if and v-show depends on the specific use case, particularly how often you expect the condition to change.

Example:

// Again, using C# for Vue.js examples is not applicable.

4. How would you create a custom directive in Vue.js that auto-focuses an input field when a page loads?

Answer: To create a custom directive in Vue.js that auto-focuses an input field when a page loads, you can define a directive that hooks into the inserted lifecycle method. This method is called when the bound element has been inserted into its parent node, ensuring the DOM is ready for manipulation.

Key Points:
- Custom directives offer a way to create reusable directives with custom logic.
- The inserted hook is ideal for DOM manipulations that depend on the element being present in the document.
- Directives have several lifecycle hooks, including bind, inserted, update, and unbind, allowing for a wide range of behaviors.

Example:

// Vue.js JavaScript example for creating a custom directive
<template>
  <input v-auto-focus>
</template>

<script>
export default {
  directives: {
    autoFocus: {
      inserted: function (el) {
        el.focus();
      }
    }
  }
}
</script>
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