Event Source Plus

A more configurable EventSource implementation that runs in browsers, NodeJS, and workers. The default browser EventSource API is too limited. Event Source Plus fixes that.

Features

Use any HTTP method
Send custom headers
Optionally change headers when retrying
Pass data as body or query params
Runs in browsers, NodeJS, and workers
First class typescript support
Automatic retry with exponential backoff and hooks for customizing behavior
ESM and CommonJS support

Installation
Usage
Listen Hooks
Contributing

Installation

# npm
npm i event-source-plus

# pnpm
pnpm i event-source-plus

Usage

Basic Request

import { EventSourcePlus } from "event-source-plus";

const eventSource = new EventSourcePlus("https://example.com");

eventSource.listen({
    onMessage(message) {
        console.log(message);
    },
});

Canceling Requests

The listen() method returns a controller that you can use to abort the request.

const controller = eventSource.listen({
    onMessage(message) {
        console.log(message);
    },
});
controller.abort();

Additional Options

The EventSourcePlus constructor allows you to pass additional fetch options such as method, body, and headers.

const eventSource = new EventSourcePlus("https://example.com", {
    method: "post",
    body: JSON.stringify({ message: "hello world" }),
    headers: {
        "Content-Type": "application/json",
    },
});

Working with Headers

Headers can be set by passing an object or a function. The function may return a header object or a promise that resolves to a header object.

// object syntax //
const eventSource = new EventSourcePlus("https://example.com", {
    // this value will remain the same for every request
    headers: {
        Authorization: "some-token",
    },
});

// function syntax //
function getHeaders() {
    return {
        Authorization: "some-token",
    };
}
const eventSource = new EventSourcePlus("https://example.com", {
    // this function will rerun every time a request is sent
    headers: getHeaders,
});

// async function syntax //
async function getHeaders() {
    const token = await getSomeToken();
    return {
        Authorization: token,
    };
}
const eventSource = new EventSourcePlus("https://example.com", {
    // this function will rerun every time a request is sent
    headers: getHeaders,
});

The function syntax is especially useful when dealing with authentication because it allows you to always get a fresh auth token. This usually a pain point when working other SSE client libraries.

Customizing Retry Behavior

By default this library will automatically retry the request indefinitely with exponential backoff maxing out at 30 seconds. Both those these values can be adjusted when initializing the EventSourcePlus class.

const eventSource = new EventSourcePlus("https://example.com", {
    // automatically retry up to 100 times (default is 'undefined')
    maxRetryCount: 100,
    // set exponential backoff to max out at 10000 ms (default is "30000")
    maxRetryInterval: 10000,
});

Additionally, you can abort the request inside listen hooks using the EventSourceController

// abort the request if we receive 10 server errors
let errCount = 0;
const controller = eventSource.listen({
    onMessage(data) {},
    onResponseError({ request, response, options }) {
        errCount++;
        if (errCount >= 10) {
            controller.abort();
        }
    },
});

Listen Hooks

The listen() method has the following hooks:

onMessage
onRequest
onRequestError
onResponse
onResponseError

The only required hook is onMessage.

`onMessage(message)`

onMessage is called whenever receiving a new Server Sent Event from the server.

eventSource.listen({
    onMessage(message) {
        console.log(message);
    },
});

`onRequest({ request, options })`

onRequest is called as soon as a request is constructed. This allows you to modify the request or do simple logging.

eventSource.listen({
    onRequest({ request, options }) {
        console.log(request, options);

        // add current time query search params
        options.query = options.query || {};
        options.query.t = new Date();
    },
});

`onRequestError({request, options, error})`

onRequestError will be called when the request fails.

eventSource.listen({
    async onRequestError({ request, options, error }) {
        console.log(`[request error]`, request, error);
    },
});

Some example errors might be Connection refused or Failed to parse URL

`onResponse({ request, options, response })`

onResponse will be called after receiving a response from the server.

eventSource.listen({
    async onResponse({ request, response, options }) {
        console.log(`Received status code: ${response.status}`);
    },
});

`onResponseError({ request, options, response })`

onResponseError will fire if one of the following conditions have been met

response.ok is not true (i.e. server returned an error status code)
The Content-Type header sent by the server doesn't include text/event-stream

eventSource.listen({
    async onResponseError({ request, response, options }) {
        console.log(
            `[response error]`,
            request,
            response.status,
            response.body,
        );
    },
});

Contributing

Pull requests and issue reports are welcome.

Before submitting a PR please ensure that you have run the following commands and there are no errors.

pnpm run lint
pnpm run format

(For VSCode users "formatOnSave" is set to true. So the formatting step may be unnecessary)

Integration tests and unit tests get run by CI.

joshmossas/event-source-plus