gas to check new gmails

checkNewMails.ts

function f() {

    const newMails = checkNewMail();

    //arbitrary operation
    newMails.forEach(console.log);

}

function checkNewMail(): GoogleAppsScript.Gmail.GmailMessage[] {

    const sheetName = 'id';
    const searchString = 'from:example@gmail.com';
    const numThread = 10;

    const idSheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName(sheetName);
    if (idSheet === null) {
        throw new Error(`Sheet [ ${sheetName} ] not found.`);
    }
    let lastRow = idSheet.getLastRow();
    if (lastRow === 0) {
        idSheet.getRange('A1:H1').setValues([['id', 'timestamp', 'subject', 'from', 'to', 'cc', 'bcc', 'body']]);
        idSheet.getRange('A1:1').setBackground('#cccccc');
        idSheet.setFrozenRows(1);
        ++lastRow;
    }

    const existingIds = idSheet.getRange(`A1:A${lastRow}`).getValues().slice(1).map(l => l[0].toString());

    const newMails = GmailApp.search(searchString, 0, numThread)
        .reduce(
            (l: GoogleAppsScript.Gmail.GmailMessage[], thread) => {
                l.push(...thread.getMessages());
                return l;
            },
            []
        )
        .filter(mail => !existingIds.includes(mail.getId()));

    if (newMails.length != 0) {
        idSheet.getRange(`A${lastRow + 1}:H${lastRow + newMails.length}`).setValues(
            newMails.map(mail => [
                mail.getId(),
                mail.getDate(),
                mail.getSubject(),
                mail.getFrom(),
                mail.getTo(),
                mail.getCc(),
                mail.getBcc(),
                mail.getPlainBody(),
            ])
        );
    }

    return newMails;

}