Iterating Rendering Looping Through The List in LWC

/ apex, Salesforce / By

To iterate through a list, we employ two directives:

  1. for:each
  2. Iterator

When utilizing for:each or Iterator, it’s crucial to include the key directive on the iterated element. The key attribute provides a unique identifier for each item. It’s important to note that iteration is impossible without the key attribute.

The framework, upon list modification, utilizes the key to selectively re-render only the item that underwent changes. This mechanism serves as a performance optimization and isn’t visibly reflected in the DOM during runtime.

for:each

When employing the for:each directive, utilize for:item=”currentItem” to access the present item. Assign a key to the initial element within the nested template using the key={uniqueId} directive. Optionally, use for:index=”index” to access the current item’s index.

Example: HelloForEach.html

<template>
    <lightning-card title="HelloForEach" icon-name="custom:custom14">
        <ul class="slds-m-around_medium">
            <template for:each={contacts} for:item="contact">
                <li key={contact.Id}>
                    {contact.Name}, {contact.Title}
                </li>
            </template>
        </ul>
    </lightning-card>
</template>

HelloForEach.js

import { LightningElement } from 'lwc';
 
export default class HelloForEach extends LightningElement {
    contacts = [
        {
            Id: 1,
            Name: 'Amy Taylor',
            Title: 'VP of Engineering',
        },
        {
            Id: 2,
            Name: 'Michael Jones',
            Title: 'VP of Sales',
        },
        {
            Id: 3,
            Name: 'Jennifer Wu',
            Title: 'CEO',
        },
    ];
}

Output:

Another Practical Example:

AccountList.hmtl

AccountList.js

AccountApex.cls

Output

iterating mechanism

For custom behavior applied to the initial or final item in a list, employ the iterator directive: iterator:iteratorName={array}. Implement this directive within a template tag.

Utilize iteratorName to access these attributes:

  • value: Represents the item’s value within the list. Access array properties using this attribute, e.g., iteratorName.value.propertyName.
  • index: Indicates the item’s index within the list.
  • first: A boolean value signaling if this item is the first entry in the list.
  • last: A boolean value signaling if this item is the final entry in the list.

Example: HelloIterator.html

<template>
    <lightning-card title="HelloIterator" icon-name="custom:custom14">
        <ul class="slds-m-around_medium">
            <template iterator:it={contacts}>
                <li key={it.value.Id}>
                    <div if:true={it.first} class="list-first"></div>
                    {it.value.Name}, {it.value.Title}
                    <div if:true={it.last} class="list-last"></div>
                </li>
            </template>
        </ul>
    </lightning-card>
</template>

helloIterator.css

.list-first {
    border-top: 1px solid black;
    padding-top: 5px;
}
 
.list-last {
    border-bottom: 1px solid black;
    padding-bottom: 5px;
}

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