jedwards1211 / repeater

The missing constructor for creating safe async iterators

Repeater.js

The missing constructor for creating safe async iterators.

For more information, visit repeater.js.org.

Installation

Repeater.js is available on npm in the CommonJS and ESModule formats.

$ npm install @repeaterjs/repeater

$ yarn add @repeaterjs/repeater

Requirements

The core @repeaterjs/repeater module has no dependencies, but requires the following globals in order to work:

• Promise
• WeakMap
• Symbol
  • Symbol.iterator
  • Symbol.asyncIterator

In addition, repeaters are most useful when used via async/await and for await…of syntax. You can transpile your code with babel or typescript to support enviroments which lack these features.

Examples

Logging timestamps with setInterval

import { Repeater } from "@repeaterjs/repeater";

const timestamps = new Repeater(async (push, stop) => {
  push(Date.now());
  const interval = setInterval(() => push(Date.now()), 1000);
  await stop;
  clearInterval(interval);
});

(async function() {
  let i = 0;
  for await (const timestamp of timestamps) {
    console.log(timestamp);
    i++;
    if (i >= 10) {
      console.log("ALL DONE!");
      break; // triggers clearInterval above
    }
  }
})();

Creating a repeater from a websocket

import { Repeater } from "@repeaterjs/repeater";

const socket = new WebSocket("ws://echo.websocket.org");
const messages = new Repeater(async (push, stop) => {
  socket.onmessage = (ev) => push(ev.data);
  socket.onerror = () => stop(new Error("WebSocket error"));
  socket.onclose = () => stop();
  await stop;
  socket.close();
});

(async function() {
  for await (const message of messages) {
    console.log(message);
    if (message === "close") {
      console.log("Closing!");
      break; // closes the socket
    }
  }
})();

socket.onopen = () => {
  socket.send("hello"); // "hello"
  socket.send("world"); // "world"
  socket.send("close"); // "close", "Closing!"
};

Listening for the Konami Code and canceling if Escape is pressed

import { Repeater } from "@repeaterjs/repeater";

const keys = new Repeater(async (push, stop) => {
  const listener = (ev) => {
    if (ev.key === "Escape") {
      stop();
    } else {
      push(ev.key);
    }
  };
  window.addEventListener("keyup", listener);
  await stop;
  window.removeEventListener("keyup", listener);
});

const konami = ["ArrowUp", "ArrowUp", "ArrowDown", "ArrowDown", "ArrowLeft", "ArrowRight", "ArrowLeft", "ArrowRight", "b", "a"];

(async function() {
  let i = 0;
  for await (const key of keys) {
    if (key === konami[i]) {
      i++;
    } else {
      i = 0;
    }
    if (i >= konami.length) {
      console.log("KONAMI!!!");
      break; // removes the keyup listener
    }
  }
})();

Converting an observable to an async iterator

import { Subject } from "rxjs";
import { Repeater } from "@repeaterjs/repeater";

const observable = new Subject();
const repeater = new Repeater(async (push, stop) => {
  const subscription = observable.subscribe({
    next: (value) => push(value),
    error: (err) => stop(err),
    complete: () => stop(),
  });
  await stop;
  subscription.unsubscribe();
});

(async function() {
  try {
    for await (const value of repeater) {
      console.log("Value: ", value);
    }
  } catch (err) {
    console.log("Error caught: ", err);
  }
})();
observable.next(1);
// Value: 1
observable.next(2);
// Value: 2
observable.error(new Error("Hello from observable"));
// Error caught: Error: Hello from observable