Template Looping in LWC

 There are 2 types of template looping in LWC

1. for:each
2. iterator

For Each

<template for:each={array} for:item="currentItem" for:index="index">

// we can add repeatable components

</template>

Note- 

1. for:each attribute take array as input

2. for item represent the current item of the loop, currentItem is the alias name, we can name to anything.

3. for:index holds index of current element of an array.

Importance of Key attribute in Template loop

Key is a special string/integer attribute required to be included to first element inside template when creating list of elements.

Key help LWC engine to identify which items added,removed or modified.

Key must be string or integer, It cant be object.

Array Index cant be defined as key.

<template>

    <lightning-card title="Best Companies">

        <div class="slds-m-around_medium">

            <template for:each={companyList} for:item="company">

                <ul key={company} class="slds-has-dividers_around-space">

                    <li class="slds-item">{company}</li>            

                    </ul>

            </template>

        </div>

    </lightning-card>

</template>

java script 

import { LightningElement } from 'lwc';

export default class Looping extends LightningElement {

    companyList = ['Google','Meta','Amazon','Salesforce','Signature Healthcare']

}

Iterator

To apply special behavior to the first or last element of list, iterator is preferable over for: each.

<template iterator:iteratorName={array}>

// here is repeating elements

</template>

Note

1. Iterator is a keyword to tell LWC engine its iterator loop.

2. IteratorName is the alias name, holds current element of array.

3. array is data on which loop is going to be executed.

Properties of Iterator

Value- the value of item of the array. Use this property to get array properties. Iteratorname.value.propertiesname

index: index of current item of array. iteratorname.index

first: Boolean(true/false) value indicates whether element is the first element of array. iteratorname.first

last: Boolean value indicates whether element is the last element of array. iteratorname.last

<template> 

<lightning-card title="CEO List">

        <div class="slds-m-around_medium">

            <template iterator:ceo={ceoList}>

                <ul key={ceo.value.id} class="slds-has-dividers_around-space">

                    <template if:true={ceo.first}>

                        <div class="slds-box slds-theme_shade">

                            <strong> List of Top compaies and Ceo Details</strong>

                        </div>

                    </template>

                    <li class="slds-item">

                        <Strong>{ceo.value.company}</Strong>                  

                        {ceo.value.name} {ceo.value.age} {ceo.value.salary}

                    </li>

                    <template if:true={ceo.last}>

                        <div class="slds-box slds-theme_shade">

                            <strong> Thank you</strong>

                        </div>

                    </template>

                    </ul>

            </template>

        </div>

    </lightning-card>

</template>

Javascript code

import { LightningElement } from 'lwc';

export default class Looping extends LightningElement {

    ceoList = [

        {id:1,name:'Sundar Pichai',age:49,salary:'2m',company:'Google'},

        {id:2,name:'Mark Zuckerberg',age:37,salary:'12B',company:'Meta'},

        {id:3,name:'Andy Jassy',age:54,salary:'35B',company:'Amazon'},

        {id:4,name:'Marc Benioff',age:57,salary:'26m',company:'salesfoce'},

        {id:5,name:'Joe Steier',age:58,salary:'500k',company:'SignatureHealthcare'},

    ]

}