Vue Computed

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Create Reusable Submit Button State Module

You’re browsing the documentation for v2.x and earlier. For v3.x, click here. In-template expressions are very convenient, but they are meant for simple operations. Putting too much logic in your templates can make them bloated and hard to maintain.

At this point, the template is no longer simple and declarative. You have to look at it for a second before realizing that it displays message in reverse. The problem is made worse when you want to include the reversed message in your template more than once.

Bind Input Field Data To The User Object

That’s why for any complex logic, you should use a computed property.

  • Original message: "{{ message }}". Computed reversed message: "{{ reversedMessage }}". Here we have declared a computed property reversedMessage.
  • The function we provided will be used as the getter function for the property vm.reversedMessage:.
  • You can open the console and play with the example vm yourself. The value of vm.reversedMessage is always dependent on the value of vm.message.

You can, the setter will be invoked and vm.firstName and vm.lastName will be updated accordingly. While computed properties are more appropriate in most cases, there are times when a custom watcher is necessary.

Longer explanation

That’s why Vue provides a more generic way to react to data changes through the watch option.

This is most useful when you want to perform asynchronous or expensive operations in response to changing data. In this case, using the watch option allows us to perform an asynchronous operation (accessing an API), limit how often we perform that operation, and set intermediary states until we get a final answer.

None of that would be possible with a computed property.

In addition to the watch option, you can also use the imperative vm.$watch API. ← Template SyntaxClass and Style Bindings →.

Caught a mistake or want to contribute to the documentation?Edit this on GitHub!Deployed onNetlify . Want to learn how to implement client-side form validation functionality in Vue js 3 Composition API for the sign-up and login pages?

Then, you’ve come to the right place! Before going any further, let’s take a look at what you’re going to learn in this Vue JS 3 Composition API tutorial.

Sign Up Button State Validation

As you can see from the final output below, all of the input fields in the sign-up form have empty check validation as well as additional validation such as email, min length etc on keyup and blur events.

  • When all the input fields are filled with no errors, the submit button is enabled, otherwise it is disabled.
  • Along the way, you’re going to learn how to create reusable and scalable code for form validation that you can use throughout your web app.

Sound interesting? Let’s get started! I assume you already know how to get Up and Running With Vue JS 3 Project Using Vue CLI.

If you’re using Options API instead of Composition API, check out the link below:Vue JS 2 Form Validation Using Options API.

I have a simple SignUp.vue component inside the src/views folder and it has a basic skeleton of a Composition API vue component.

Some CSS style at the bottom makes the form center to the viewport horizontally and vertically. Rather than creating template code for the name input field in the SignUp.vue file, create a new child component called NameField.vue inside the src/components folder.

As you can see, I’ve used some additional CSS classes that are coming from Semantic UI.

You do not need to know this in order to follow along with the rest of this guide.

Feel free to use your favourite CSS framework. Let’s import the NameField.vue child component to the Signup.vue with three simple steps.

1 import NameField.vue child components inside the Signup.vue file.

2 Register it by adding it to the components object.

3 Finally, call it in the template. And the output will look like this:. Pretty straight forward! Now, we’re ready to do the validation for the Name Input Field. First, I’m going to check if the input field is empty or not using computed property inside the NameField.vue component.

Declare a variable called input initialized with an empty string using ref() object inside the setup() function. RecommendedDifferences Between Ref() Vs Reactive() in Vue 3 Compoisition API. Bind it to the name input field at the top in the vue template.

Now, define a computed property called error which will return an error message if the value of the input field is empty, otherwise return an empty string.

Finally, add the error variable to the returned object to show the error message on the view.

Now, let add some markup to show the error message.

As I mentioned earlier, the additional CSS classes in the markup come from the Semantic UI CSS Framework. Also check to make sure to only show the error message if the error variable is not empty using the v-if directive.

The next step: use LogRocket to see a replay of Vuex mutations for every user in your app

This works fine…. But the error message is visible by default…. What we want is to not show any error until the user starts interacting with the input field.

To fix it, set the default value of the input variable to null instead of “” . That’s works great! So what’s happening…? When the component loads, the value of the input is set to null so the input.value“” inside the error computed property is false which hides the error message by default.

When a user starts typing and clearing all of the characters, the error computed property returns the error message which then will be visible to a user.