Render lists

Completed

Working with data often involves arrays or other types of collections. You ordinarily iterate through all items in a collection by using some form of loop. Vue.js supports loops through a directive called v-for.

Introducing the v-for directive

To display all items in a list, you can use the directive v-for. v-for behaves much like a for...of loop in JavaScript. It iterates through a collection, providing access to each item via the variable you declare.

The syntax for v-for looks like:

v-for="itemName in collectionName"

collectionName is the name of the array in your data object. For each iteration, itemName assumes the identity of your current item. To use v-for in a template, you add the directive inside an element declaration. Doing so repeats the HTML element as many times as there are items in the array.

Note

If you're creating an unordered list (ul), you would not add v-for to the ul element. You add it to the li element, because the li element is what will be repeated.

Use v-for in an app

To use the v-for directive in an app, you need to do two things:

• Expose an array via your data object. To use the v-for directive, ensure that there's data of type array that your data() method is returning.
• Add the v-for directive to an element. Add the v-for directive to an HTML element. To display data from the array, use double braces ({{ }}) syntax.

Let's say you want to expose an array of names in your data object:

Vue.createApp({
    data() {
        return {
            names: ['Susan', 'Peter', 'Bill' ]
        }
    }
}).mount('#demo');

If you want to display the items, you could use the following code:

<ul id="demo">
    <li v-for="name in names">{{ name }}</li>
</ul>

When the code is rendered, all the names in the array are displayed:

• Susan
• Peter
• Bill

Keys and state

If the data is modified, Vue.js needs to be able to refresh the appropriate parts of the display. When you're working with a list, it's best to update the individual item rather than the entire list. To allow Vue.js to find an individual item, give each displayed item a key. You don't have to add the key to your data object; you can generate it by using the index of the array.

You can obtain the index by updating the v-for declaration, as shown in the following code:

<ul id="demo">
    <li v-for="(name, index) in names" :key="index">{{ name }}</li>
</ul>

index is incremented for each item in the array. The :key directive stores the key for Vue.js, which allows it to update the individual item as changes occur.