It's a peach

Cross-Origin Resource Sharing

Cross-Origin Resource Sharing (CORS) is a way for a server to allow a client to access it's data in a browser.

To demonstrate the functionality we will need two web servers. One to provide data, and a second to request the data.

The source code for the REST server can be found here (and here here with typing).

The source code for the web server can be found here (and here here with typing) with the html here.

REST Server

There is very little required to add CORS support. All you need to do is add the middleware.

from bareasgi import Application, text_reader, text_writer
from bareasgi_cors import CORSMiddleware

cors_middleware = CORSMiddleware()

app = Application(

POST requests

When a browser makes a cross origin POST it will first make an OPTIONS request. Some web frameworks will transparently handle this, but in the "bare" tradition of this framework it is left to the developer to decide this.

However, typically, if you have added CORS support you probably wanted to add the OPTIONS method to any POST route.

cors_middleware = CORSMiddleware()

app = Application(
    info={'name': 'Michael Caine'},

app.http_router.add({'GET'}, '/info', get_info)
app.http_router.add({'POST', 'OPTIONS'}, '/info', set_info)

Note that this is not required on a GET.

Web Server

The web server doesn't need to do anything special.

The page the web server provides calls a GET when the page loads.

window.onload = function() {
    .then(function(response) {
    return response.json();
    .then(function(info) {
    const element = document.getElementById('info');
    element.value =;

And when the form is submitted it makes a POST.

fetch('', {
    method: 'POST',
    headers: {
    'Content-Type': 'application/json'
    body: JSON.stringify(data)
    .then(function(response) {
    return Promise.resolve('Done');

You can check that it's working by commenting out the CORS middleware in the REST server. The browser will reject the fetch requests.

What next?

Either go back to the table of contents or go to Diversion: A Simple REST Blog.