How to Use Computed Properties in Vue: Simple Guide

In Vue, computed properties are functions that automatically update when their dependencies change and cache their results for better performance. You define them inside the computed option in your component to create reactive values based on other data.

Syntax

The computed option in Vue is an object where each key is a computed property name and the value is a function that returns the computed value. Vue tracks dependencies inside this function and updates the value only when needed.

Example parts:

• computed: The object holding computed properties.
• propertyName(): A function returning the computed value.

javascript

export default {
  data() {
    return {
      firstName: 'John',
      lastName: 'Doe'
    };
  },
  computed: {
    fullName() {
      return this.firstName + ' ' + this.lastName;
    }
  }
};

Example

This example shows a Vue component that uses a computed property fullName to combine firstName and lastName. When either changes, fullName updates automatically and efficiently.

vue

<template>
  <div>
    <input v-model="firstName" placeholder="First Name" />
    <input v-model="lastName" placeholder="Last Name" />
    <p>Full Name: {{ fullName }}</p>
  </div>
</template>

<script>
export default {
  data() {
    return {
      firstName: 'Jane',
      lastName: 'Smith'
    };
  },
  computed: {
    fullName() {
      return `${this.firstName} ${this.lastName}`;
    }
  }
};
</script>

Output

Full Name: Jane Smith

Common Pitfalls

Common mistakes when using computed include:

• Trying to set a value directly on a computed property without a setter (computed properties are read-only by default).
• Using methods instead of computed properties when caching is needed, causing unnecessary recalculations.
• Not returning a value from the computed function.

javascript

export default {
  data() {
    return { count: 1 };
  },
  computed: {
    // Wrong: trying to assign value directly
    doubleCount: 2 * this.count
  }
};

// Correct way:
export default {
  data() {
    return { count: 1 };
  },
  computed: {
    doubleCount() {
      return this.count * 2;
    }
  }
};

Quick Reference

• Use computed for values derived from reactive data that need caching.
• Computed properties are cached until dependencies change.
• Define computed as functions inside the computed object.
• Computed properties are read-only unless you add a setter.

Key Takeaways

Use computed properties to create reactive, cached values based on component data.

Define computed properties as functions inside the computed object in your Vue component.

Computed properties update automatically when their dependencies change and cache results for performance.

Avoid trying to assign values directly to computed properties unless you define a setter.

Use computed properties instead of methods when you want caching and reactive updates.