Conditional Directives With Vue.js

The ability to show or hide elements based on conditions is a fundamental feature of any frontend framework. Vue.js provides us with a set of core directives to achieve this effect: v-if, v-else, v-else-if and v-show.

After getting started with a Vue.js Hello World application, we can use values from JavaScript data object to conditionally control the view. For example, we’ll see how to use the simple data object given below.

data() {
  return {
    msg: "Hello World!",
    isLoggedIn: false
  }
}

v-if

The v-if directive adds or removes DOM elements based on the given expression.

We can use the isLoggedIn property from the data model and show a login button in the view.

<button v-if="isLoggedIn">Logout</button>

Now, the button will not show because isLoggedIn is set to false. Setting the data.isLoggedIn value to true would add the button to the DOM.

<template> element

The v-if directive can only show or hide one element (and its child elements), but you can also control multiple elements with a single v-if reducing duplication.

To do this, you need to wrap all the elements that should be controlled by this condition in a <template> element. The template element itself will not be added to the DOM. But all containing elements will be added or removed depending on the v-if expression.

For example, if you need to show a label as well as a button when the isLoggedIn is true you can wrap both elements in a single template element as follows.

<template v-if="isLoggedIn">
  <label> Logout </button>
  <button> Logout </button>
</template>

v-else

As the name v-else suggests, this directive is used to display content only when the expression adjacent v-if resolves to false.

We can have a Log In button to show automatically when the isLoggedIn is false.

<button v-if="isLoggedIn"> Logout </button>
<button v-else> Log In </button>

v-else does not need a value passed in to it. But it must be in an element that comes immediately after an element containing v-if or v-else-if directives.

v-else-if

v-else-if can be used when we need more than two options to be checked. This will ensure that only one of the chained items in the else-if chain will be visible.

For example, if the property named isLoginDisabled is true, we can prevent the Log In button from displaying and instead display a label. We can accomplish it by using the v-else-if directive as follows.

<button v-if="isLoggedIn"> Logout </button>
<label v-else-if="isLoginDisabled"> Register disabled </label>
<button v-else> Log In </button>

v-show

Very similar to v-if, the v-show directive can also be used to show and hide an element based on an expression.

The main difference between the two is that,

• v-if - Only renders the element to the DOM if the expression passes.
• v-show - Renders all elements to the DOM and then uses the CSS display property to hide elements if the expression fails.
• v-show - Does not support v-else, v-else-if

Usually, v-show has a performance advantage if the elements are switched on and off frequently, while the v-if has the advantage when it comes to initial render time.