Why do we need getter in LWC?

 LWC template/component does not allow expression or any manipulation or array expression. To solve this problem "Getter" is solution. It atomically bind data from Java scripts to template.

Error = Invalid expression {employee[0]} - LWC1038: Template expression doesn't allow computed property accesslwc

 <div class="slds-m-around_medium">
           {employee[0]}
<p> The sum of {num1} and {num2} is {num1+num2}
 </div>

Component code

<template>

    <lightning-card title="Getters in LWC">

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

            {firstEmployee}

            <p> The sum of {num1} and {num2} is {sumResults}</p>

        </div>

    </lightning-card>

</template>

Java script code

When employee gets updated then automatically firstemployee getter called and updated value will be reflected in component.

import { LightningElement } from 'lwc';

export default class HelloWorld1 extends LightningElement {

   

    num1 = 10;

    num2 = 20;

    employee = ["Asish","behera","Ipsheeta"]

    get firstEmployee(){

        return this.employee[0].toUpperCase();

    }

    get sumResults(){

        return this.num1+this.num2;

    }

   

}

Why do we need track decorator in LWC?

 LWC engine can track changes or value assigned to a field or primitive properties automatically, however if field/properties is array or object type then LWC will not be able track value change automatically. 

That is when @track is helpful, this will tell LWC engine to observe changes to the properties of object or elements of array decorated with @track.

Here is my component code

We  can also achieve using spread operator, the key here is instead of mutating same object/array we can create copy and make changes. LWC framework observes value assigned to field/property and if new value !=== previous value then component re-renders.

this.address = {...this.address,"city":event.target.value}

<template>

    <lightning-card title="2 way Data Binding">

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

            <lightning-input type="text" label="Enter course Name"

                onkeyup={changehandler}></lightning-input>

            <div>{fullname} is course of {title}</div>            

        </div>

    </lightning-card>

    <lightning-card title="@track properties">

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

            <lightning-input type="text" label="Enter City Name"

                onkeyup={trackhandler}></lightning-input>

            <div>{address.city} is my city</div>            

        </div>

    </lightning-card>  

</template>

Java script code

import { LightningElement,track } from 'lwc';

export default class HelloWorld1 extends LightningElement {

    fullname = "LWC Learning"

    title ="aura"

   

     @track address = {

        city:"Louisville",

        postalCode:40291,

        country:"USA"

    }

    @track hobbies = ['Music','Reading','Netflix'];

    changehandler(event){

        this.title = event.target.value;

    }

    trackhandler(event){

        //this.address = {...this.address,"city":event.target.value}

        this.address.city=event.target.value;

    }

   

}

How to retrieve populated fields from a sObject ?

 As part of summer 16, the sObject method getPopulatedFieldsAsMap() was released. This method will return a map of populated field names and their corresponding values. The map contains only the fields that have been populated in memory for the SObject instance.

https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_sobject.htm#apex_System_SObject_getPopulatedFieldsAsMap

Here are certain use cases to  this method.

1.  Fields are queried in the SOQL Statement

Account acc = [SELECT id,name,phone,Gender__pc From Account

               WHERE id = '0011F00000zJIvwQAG'];

System.debug('acc->'+acc);

Map<String, Object> accMap = acc.getPopulatedFieldsAsMap();

system.debug('Fields name-->'+accMap.keySet());

for (String fieldName : accMap.keySet()){

    System.debug('field name is ' + fieldName + ', value is ' + 

        accMap.get(fieldName));

}

OutPut -

Note - Id, and Recodtype will come by default even if it is not added in Query.

We can also use dynamic query for this.

public static void getPopulatedFields(List<String> fields, String objName) {

    String soql = ' Select ' + String.join(fields, ',') + 

                  ' From ' + objName;

    

    for (sObject obj : Database.query(soql)) {

       Map<String, Object> fieldValues = obj.getPopulatedFieldsAsMap();

       for (String fieldName : fieldValues.keySet()) {

           System.debug(fieldName + '=' + fieldValues.get(fieldName));

       }

    } 

}

// Invoking method

List<String> fields = new List<String>{'Name','Phone','Gender__pc'};

getPopulatedFields(fields, 'Account');

2. Fields are explicitly set before calling the getPopulatedFieldsAsMap() method.

Account acc = new Account(Name='Test Acc',Phone='1122334565');

//sert acc;

Map<String, Object> accMap = acc.getPopulatedFieldsAsMap();

system.debug('Fields name-->'+accMap.keySet());

for (String fieldName : accMap.keySet()){

    System.debug('field name is ' + fieldName + ', value is ' + 

        accMap.get(fieldName));

}

OutPut -

3. Using Relationship Query. Child to parent.

Account acc = [SELECT id,name,phone,Gender__pc,owner.Name,Createdby.Email,

SHC_Facility__pr.Facility_Short_Name__c,(Select CaseNumber, Status From Cases)  

               FROM Account

               WHERE id = '0011F00000zJIvwQAG'];

Map<String, Object> accMap = acc.getPopulatedFieldsAsMap();

system.debug('Fields name-->'+accMap.keySet());

It will print -

13:50:17:036 USER_DEBUG [6]|DEBUG|Fields name-->

{CreatedBy, CreatedById, Gender__pc, Id, Name, Owner, OwnerId, Phone, RecordTypeId,

SHC_Facility__pc, ...}

How will we get fields of related objects such as Owner or CreatedBy or

SHC_facility__pr

User uRec = (User)accMap.get('Owner'); Map<String, Object> userMap = uRec.getPopulatedFieldsAsMap(); system.debug('Owner Fields name-->'+userMap.keySet()); User createdByRec = (User)accMap.get('CreatedBy'); Map<String, Object> CreateduserMap = createdByRec.getPopulatedFieldsAsMap(); system.debug('Created By Fields name-->'+CreateduserMap.keySet()); Rehabilitation_Center__c facility = (Rehabilitation_Center__c)accMap.get('SHC_Facility__pr'); Map<String, Object> FaciLityMap = facility.getPopulatedFieldsAsMap(); system.debug('FaciLityMap Fields name-->'+FaciLityMap.keySet());

// To get case record fields and value

List<Case> caseList = (List<Case>)accMap.get('Cases');

System.debug('caseList--'+caseList);

Map<String, Object> caseMap = caseList[0].getPopulatedFieldsAsMap();

System.debug('caseMap-fields-'+caseMap.keySet());

To know value we need to iterate over map keyset and get value.

We can also write generic code which will support all objects.

/* method to expand map of key to find out parent or child objects and get value 

and field name after

*/

public void getPopulatedFields(Map<String, Object> fieldValues) {

   for (String fieldName : fieldValues.keySet()) {

       if (fieldValues.get(fieldName) instanceof List<sObject>) {

           System.debug('------Begin Child------');

           getPopulatedFields((List<sObject>) fieldValues.get(fieldName));

           System.debug('------End Child------');

       } else if (fieldValues.get(fieldName) instanceof sObject) {

           System.debug('------Begin Parent------');

           sObject obj = (sObject) fieldValues.get(fieldName);

           getPopulatedFields(obj.getPopulatedFieldsAsMap());

           System.debug('------End Parent------');

       } else {

           System.debug(fieldName + '=' + fieldValues.get(fieldName));

       }

   }

}

/* method to iterarte over child recordList and call getPopulatedFields passing result

 of getPopulatedFieldsAsMap

*/

public void getPopulatedFields(List<sObject> objs) {

   for (sObject obj : objs) {

        getPopulatedFields(obj.getPopulatedFieldsAsMap());

    }

}

/* method will accept object name and fields name, get records by using dynamic query

 and call getPopulatedFields

*/

public void getPopulatedFields(List<String> fields, String objName) {

    String soql = ' Select ' + String.join(fields, ',') + 

                  ' From ' + objName;

    List<sObject> objs = Database.query(soql);

    getPopulatedFields(objs);

}

Invoking methods.

List<String> fields = new List<String>

{'Name,phone,Gender__pc,owner.Name,Createdby.Email,

SHC_Facility__pr.Facility_Short_Name__c, (Select CaseNumber, Status From Cases)'};

getPopulatedFields(fields,'Account')