How to sign this EOS transaction provided with below code and response?

This below code is working fine in testnet. I want to do transaction, signing and broadcasting individually. I generated the raw transaction, but I don't understand how to sign it. I have provided the real working code and current console result.

const { Api, JsonRpc } = require('eosjs')
const { JsSignatureProvider } = require('eosjs/dist/eosjs-jssig') // development only
const fetch = require('node-fetch') // node only
const { TextDecoder, TextEncoder } = require('util') // node only
const signatureProvider = new JsSignatureProvider([
    '5KAgjsrSrsXrsnxrpYY5ApAikfAEXYEVuvVhnakCQeS2KCb7dKq'
])
const rpc = new JsonRpc('http://jungle2.cryptolions.io:80', { fetch })
const ecc = require('eosjs-ecc')

const api = new Api({
    rpc,
    signatureProvider,
    textDecoder: new TextDecoder(),
    textEncoder: new TextEncoder()
})
const transaction = {
    actions: [
        {
            account: 'eosio.token',
            name: 'transfer',
            authorization: [
                {
                    actor: 'useraaaaaaax',
                    permission: 'active'
                }
            ],
            data: {
                from: 'useraaaaaaax',
                to: 'useraaaaazzz',
                quantity: '2.0000 EOS',
                memo: '25.11.2019 from x to zzz at'
            }
        }
    ]
}
const optionaldetails = {
    blocksBehind: 3,
    expireSeconds: 30,
    broadcast: false, //if this is true
    sign: false //if this is also true
    //then transaction is happening 
    //result can be seen here
    //click accountinfo and enter username either useraaaaaaax  or   useraaaaazzz
    // https://api.monitor.jungletestnet.io/#accountInfo
}


//for transaction
async function forTransaction() {
    console.log('inside transaction')
    let transactResult = await api.transact(transaction, optionaldetails)
    console.log('transactResult', transactResult)
}
forTransaction()

//console result
    inside transaction
    transactResult {
      serializedTransaction: Uint8Array [
        172,  99, 221,  93,  18, 167, 149, 245,  10, 150,   0,   0,
          0,   0,   1,   0, 166, 130,  52,   3, 234,  48,  85,   0,
          0,   0,  87,  45,  60, 205, 205,   1, 208, 141,  49, 198,
         24, 115,  21, 214,   0,   0,   0,   0, 168, 237,  50,  50,
         60, 208, 141,  49, 198,  24, 115,  21, 214, 240, 255,  55,
        198,  24, 115,  21, 214,  32,  78,   0,   0,   0,   0,   0,
          0,   4,  69,  79,  83,   0,   0,   0,   0,  27,  50,  53,
         46,  49,  49,  46,  50,  48,  49,  57,  32, 102, 114, 111,
        109,  32, 120,  32,
        ... 10 more items
      ],
      signatures: []
    }