<template>
    <lightning-card title="Export Data as CSV in Lightning Web Components" icon-name="custom:custom63">
        <template if:true={data}>
            <div class="slds-p-around_medium lgc-bg-inverse">
                <p>Do you want download data as a CSV format, Click Here &nbsp;
                    <lightning-button icon-name="utility:download" label="Download as CSV" title="Download CSV File"
                        onclick={downloadCSVFile} variant="brand"></lightning-button>
                </p>
            </div>
            <div class="slds-m-around_medium">
                <!-- Datatable component -->
                <lightning-datatable columns={columns} data={data} hide-checkbox-column="true" key-field="id">
                </lightning-datatable>
            </div>
        </template>
    </lightning-card>
</template>

import { LightningElement, track} from 'lwc';
// importing accounts
import getAccountList from '@salesforce/apex/LWCExampleController.getAccounts';
// imported to show toast messages
import {ShowToastEvent} from 'lightning/platformShowToastEvent';


// datatable columns
const cols = [
    {label: 'Name',fieldName: 'Name'}, 
    {label: 'Industry',fieldName: 'Industry'},
    {label: 'Type',fieldName: 'Type'}, 
    {label: 'Phone',fieldName: 'Phone',type: 'phone'}, 
    {label: 'Rating',fieldName: 'Rating'}, 
    {label: 'Account Number',fieldName: 'AccountNumber'}, 
];

export default class ExportDataToCSVInLWC extends LightningElement {
    @track error;
    @track data;
    @track columns = cols;

    // this constructor invoke when component is created.
    // once component is created it will fetch the accounts
    constructor() {
        super();
        this.getallaccounts();
    }

    // fetching accounts from server
    getallaccounts() {
        getAccountList()
        .then(result => {
            this.data = result;
            this.error = undefined;
        })
        .catch(error => {
            this.error = error;
            this.dispatchEvent(
                new ShowToastEvent({
                    title: 'Error while getting Accounts', 
                    message: error.message, 
                    variant: 'error'
                }),
            );
            this.data = undefined;
        });
    }

    // this method validates the data and creates the csv file to download
    downloadCSVFile() {   
        let rowEnd = '\n';
        let csvString = '';
        // this set elminates the duplicates if have any duplicate keys
        let rowData = new Set();

        // getting keys from data
        this.data.forEach(function (record) {
            Object.keys(record).forEach(function (key) {
                rowData.add(key);
            });
        });

        // Array.from() method returns an Array object from any object with a length property or an iterable object.
        rowData = Array.from(rowData);
        
        // splitting using ','
        csvString += rowData.join(',');
        csvString += rowEnd;

        // main for loop to get the data based on key value
        for(let i=0; i < this.data.length; i++){
            let colValue = 0;

            // validating keys in data
            for(let key in rowData) {
                if(rowData.hasOwnProperty(key)) {
                    // Key value 
                    // Ex: Id, Name
                    let rowKey = rowData[key];
                    // add , after every value except the first.
                    if(colValue > 0){
                        csvString += ',';
                    }
                    // If the column is undefined, it as blank in the CSV file.
                    let value = this.data[i][rowKey] === undefined ? '' : this.data[i][rowKey];
                    csvString += '"'+ value +'"';
                    colValue++;
                }
            }
            csvString += rowEnd;
        }

        // Creating anchor element to download
        let downloadElement = document.createElement('a');

        // This  encodeURI encodes special characters, except: , / ? : @ & = + $ # (Use encodeURIComponent() to encode these characters).
        downloadElement.href = 'data:text/csv;charset=utf-8,' + encodeURI(csvString);
        downloadElement.target = '_self';
        // CSV File Name
        downloadElement.download = 'Account Data.csv';
        // below statement is required if you are using firefox browser
        document.body.appendChild(downloadElement);
        // click() Javascript function to download CSV file
        downloadElement.click(); 
    }

}

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="exportDataToCSVInLWC">
    <apiVersion>45.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__AppPage</target>
        <target>lightning__RecordPage</target>
        <target>lightning__HomePage</target>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>