Netlify functions - continue after completion

Netlify functions are a powerful beast for all your API needs. But what if I want to return as soon as possible from my API call to keep my web UI responsive, but still need to do something like sending out an e-mail? Enter the "classic" function style that makes this possible.

https://docs.netlify.com/functions/overview/ can be used used to provide a API for your web site. You can buid your functions with JavaScript.

There are two common ways to write your Netlify function. The "modern" style and the "legacy" style:

// modern JS style - encouraged
export async function handler(event, context) {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: `Hello world ${Math.floor(Math.random() * 10)}` })
  };
}

// legacy callback style - not encouraged anymore, but you'll still see examples doing this
exports.handler = function(event, context, callback) {
// your server-side functionality
callback(null, {
  statusCode: 200,
  body: JSON.stringify({
    message: `Hello world ${Math.floor(Math.random() * 10)}`
  })
});
};

I would encourage you to use the modern style, except when you want to return a result as soon as posible, but continue processing in the function. This can only be done with the legacy style, as follows:

function continueAfterDoneTest.js:

function getTime() {
  return (new Date()).toISOString().substr(11, 8);
}

exports.handler = (event, context, callback) => {
  console.log(`${getTime()}: THE FUNCTION HAS STARTED`);
  setTimeout(function () {
    console.log(`${getTime()}: THE TIMEOUT IS COMPLETED`)
  }, 5000);

  callback(null, { statusCode: 200, body: `${getTime()}: FUNCTION COMPLETED` });
};

Function output in the terminal:

◈ Creating Live Tunnel for 8ddb4b97-be43-46aa-855c-0d05999a8ec9

   ┌────────────────────────────────────────────────────────────┐
   │                                                            │
   │   ◈ Server now ready on https://idnv-3ea026.netlify.live   │
   │                                                            │
   └────────────────────────────────────────────────────────────┘

Request from ::1: GET /.netlify/functions/continueAfterDoneTest
12:40:00: THE FUNCTION HAS STARTED
Response with status 200 in 8 ms.
12:40:05: THE TIMEOUT IS COMPLETED

Output in the browser at url https://idnv-3ea026.netlify.live/.netlify/functions/continueAfterDoneTest:

12:40:00: FUNCTION COMPLETED

You will see this output in the browser directly, while the last line in the terminal output comes 5 seconds later.