Today i am going to share how to create a field dependency in LWC component.
We are using Custom fields on Account that are Upsell(UpsellOpportunity__c) and SLA(SLA__c). We created the field dependency on Object level using these field as shown in image below.

So now we are all set to work LWC Component:
Html:
<template>
<lightning-combobox name="Upsell"
label="Upsell"
placeholder="Select Upsell"
options={upsellOptions}
onchange={handleUpsellChange}>
</lightning-combobox>
<lightning-combobox name="Sla"
label="Sla"
placeholder="Select Sla"
options={slaOptions}>
</lightning-combobox>
</template>
JS
import { LightningElement,wire, api, track } from 'lwc';
import {getObjectInfo} from 'lightning/uiObjectInfoApi';
import ACCOUNT_OBJECT from '@salesforce/schema/Account';
import {getPicklistValues} from 'lightning/uiObjectInfoApi';
import SLA_FIELD from '@salesforce/schema/Account.SLA__c';
import UPSELL_FIELD from '@salesforce/schema/Account.UpsellOpportunity__c';
export default class Testdependentpicklist extends LightningElement {
@wire(getObjectInfo, {objectApiName: ACCOUNT_OBJECT })
accountInfo;
@track slaOptions;
@track upsellOptions;
@wire(getPicklistValues, {recordTypeId: '$accountInfo.data.defaultRecordTypeId', fieldApiName: SLA_FIELD })
slaFieldInfo({ data, error }) {
if (data) this.slaFieldData = data;
}
@wire(getPicklistValues, {recordTypeId:'$accountInfo.data.defaultRecordTypeId', fieldApiName: UPSELL_FIELD })
upsellFieldInfo({ data, error }) {
if (data) this.upsellOptions = data.values;
}
handleUpsellChange(event) {
let key = this.slaFieldData.controllerValues[event.target.value];
this.slaOptions = this.slaFieldData.values.filter(opt => opt.validFor.includes(key));
}
}
XML File :
<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
<apiVersion>48.0</apiVersion>
<isExposed>true</isExposed>
<targets>
<target>lightning__AppPage</target>
<target>lightning__RecordPage</target>
<target>lightning__HomePage</target>
</targets>
</LightningComponentBundle>
Working Demo:

Please have a look and let me know the feedbacks .