Expression Functions in LWC?

Question

How can expression functions, similar to those used in Visualforce and Lightning Aura Components, be implemented within Lightning Web Components (LWC)?

For instance, in the following code, the expression index % 5 == 0 does not compile in LWC:

accountList.html

<template>
    <template if:true={accounts.data}>
        <template for:each={accounts.data} for:item="item" for:index="index">
            <!-- LWC does not support expression functions -->
            <!--<template if:true={index % 5 == 0}><br></template>-->
            {item}
        </template>
    </template>
</template>

accountList.js:

import { LightningElement, wire } from 'lwc';
import getAccounts from '@salesforce/apex/TestController.getAccounts';

export default class AccountList extends LightningElement {
    @wire(getAccounts) accounts;
}

Since LWC does not support expression functions, how can I implement similar functionality, such as inserting a <br> tag every 5th item in the list?

Answer

There are multiple ways to implement the desired logic in LWC. Since all computations are handled in JavaScript and not in the markup, here are two possible approaches:

Option 1: Using a Dedicated Component for Each Item

You can create a dedicated component to handle the display logic for each item in the list. This way, you can manage the computation of whether the index is a multiple of 5 inside the child component.

For example, you can create a new component called AccountListItem and implement the logic as follows:

import { LightningElement, api } from 'lwc';

export default class AccountListItem extends LightningElement {
   @api index;
   @api item;

   get isMod5() {
     return this.index % 5 === 0;
   }
}

Then, in your main component (e.g., AccountList), you would use this AccountListItem component for each item and pass the index and item as parameters.

Option 2: Manipulating Data Using a Wired Function

Instead of directly using a wired property, you can use a wired function to manipulate the data as it’s returned from the Apex controller. You can modify the data model to add an isMod5 property that checks if the index is a multiple of 5.

Here’s an example of how this could be implemented:

import { LightningElement, wire, track } from 'lwc';
import getAccounts from '@salesforce/apex/TestController.getAccounts';

export default class AccountList extends LightningElement {
    @track accounts;

    @wire(getAccounts)
    wiredAccounts({ error, data }) {
       if (error) {
         // Handle error
       } else if (data) {
         this.accounts = data.map((acct, index) => {
            return {
               acct: acct,
               isMod5: index % 5 === 0
            };
         });
       }
    }
}

In this approach, the accounts property is modified when the data is fetched, and each account object is supplemented with an isMod5 property, which indicates whether the item’s index is a multiple of 5. You can then use this property in your template to conditionally render content.

Both options offer ways to implement logic in LWC to achieve similar behavior as expression functions in Visualforce or Aura Components.

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