What is the use of foreach statement in vue

Vue.js is a popular front-end framework, and its excellent performance has attracted more and more developers. In Vue.js, the foreach statement is widely used to iterate over data in arrays and objects. Below, we will introduce in detail how to use the foreach statement in Vue.js.

1. Traverse the array

When the foreach statement in Vue.js is used to traverse the array, it is usually implemented using the v-for instruction. The v-for directive is usually placed on the HTML tags that need to be traversed. The specific usage is as follows:

<div>
    <ul>
        <li v-for="item in items">{{ item }}</li>
    </ul>
</div>

In the above code, the v-for instruction traverses each element in the items array and renders the value of each element into the text content of the li tag. During the iteration, we use the item variable to reference each element in the array.

During the process of traversing the array, Vue.js provides us with an optional second parameter, which can be used to reference the index value of the element. For example:

<div>
    <ul>
        <li v-for="(item, index) in items">{{ index }}：{{ item }}</li>
    </ul>
</div>

In the above code, we use a variable named index to refer to the index value of the array element, and at the same time render it into the li tag together with the array element value.

2. Traverse objects

Unlike arrays, the special syntax form of the v-for directive must be used when traversing objects. The following is a simple example:

<div>
    <ul>
        <li v-for="(value, key) in myObject">{{ key }}：{{ value }}</li>
    </ul>
</div>

In the above code, we use the myObject object as the traversal target, and use the value and key variables to reference the value and key in the object respectively.

Similar to the second parameter used for arrays, we can optionally use a third parameter to provide a Boolean value about whether the current item is an object, as follows:

<div>
    <ul>
        <li v-for="(value, key, index) in myObject">{{ index }}：{{ key }}={{ value }}</li>
    </ul>
</div>

In the above code, we use the index variable to refer to the index value of the current item and render it into the li tag together with the object key value.

3. Traverse the properties of an object

In Vue.js, you can also use the v-for directive to traverse the properties of an object. The following is a simple example:

<div>
    <ul>
        <li v-for="prop in myObject">{{ myObject[prop] }}</li>
    </ul>
</div>

In the above code, we use a prop variable to reference the object property name, and use the [] operator to obtain the object property value. In addition, we can also specify a filter condition during traversal and only render the attribute values ​​that meet the conditions:

<div>
    <ul>
        <li v-for="prop in myObject" v-if="myObject[prop] > 18">{{ myObject[prop] }}</li>
    </ul>
</div>

In the above code, we only render the attributes in the myObject object whose attribute value is greater than 18.

4. Add key when traversing

Finally, it should also be noted that when using the v-for directive to traverse an array or object, Vue.js requires that each traversal Items must have a unique key attribute. The purpose of this key attribute is to help Vue.js locate changed traversal items faster when rendering the template.

The following is a specific example:

<div>
    <ul>
        <li v-for="(item, index) in items" :key="index">{{ item }}</li>
    </ul>
</div>

In the above code, we use the :key binding syntax to specify a key value. Generally, it is recommended to use a unique string as the key value rather than the index value of an array or object.

Conclusion

In Vue.js, we can use the foreach statement to iterate through the data of arrays and objects, and then use the v-for directive to render it into an HTML page. When using the v-for directive, we need to pay attention to adding a unique key attribute to each traversed item when traversing the array or object to help Vue.js track data changes more accurately. In addition, we can also use the second and third parameters to refer to the index value of an array or object or to determine whether the current item is an object.

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