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Netlify functions - continue after completion

September 21, 2020 • ☕️ 2 min read 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, and it is required for returning a response to the requester and allowing the function to continue its process. If you use the “legacy” style the response of the callback will not be sent to the requester until the function is done with all invocations or it times out at 10 seconds, even when the setTimeout technique is used as illustrated in the following “modern” style example.

Use the “modern” style like so:

function continueAfterDoneTest.js:

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

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

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

Function output in the terminal:

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

   │                                                            │
   │   ◈ Server now ready on   │
   │                                                            │

Request from ::1: GET /.netlify/functions/continueAfterDoneTest
Response with status 200 in 8 ms.

Output in the browser at url


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

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Serge van den Oever's weblog

Serge van den Oever

Personal blog by Serge van den Oever - als je maar lol hebt...
Twitter: @svdoever
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Technology Consultant @ Macaw
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