polar

A tiny flexible JS polling library with typescript support.

Installation

npm install js-polar

Getting Started

import { Polar } from "js-polar";

const polar = new Polar({
    request: () => fetch("https://github.com"),
    onPoll: (response, actions, properties) => {
        if (response.body.completed) {
            actions.stop();
        }
    }
});

polar.start().catch(myHandleErrFunction);

If you don't want to use ES6 imports, you can use const { Polar } = require("js-polar").

Constructor Options

| Name | Description | Default | | -------------------- | :-------------------------------------------------------------------------------- | :-------------- | | request | The poll request. | None (required) | | beforePoll | Lifecycle method to execute before attempting each poll. | () => {} | | onPoll | Lifecycle method to execute upon each successful poll. | () => {} | | afterPoll | Lifecycle method to execute after attempting each poll. | () => {} | | delay | Delay in ms between each poll. | 2000 | | limit | Maximum number of polls before stopping. | null (no limit) | | maxRetries | Retry on a failed response a set number of times before exiting the poll process. | 0 | | continueOnError (*) | Continue polling indefinitely when an error response is received. | false |

* Note that if continueOnError is true, any catch block attached to the polar.start() call will not pick up errors. Instead, the error can be accessed via properties.error inside the lifecycle methods.

Lifecycle Parameters

Below are type definitions for lifecycle methods:

onPoll: (response, actions, properties) => void

beforePoll: (actions, properties) => void

afterPoll: (actions, properties) => void

response

The response object returned by the most recent poll.

actions

An object containing actions which allow the user to stop the poll process or update the initial poll parameters from within lifecycle methods. These are actions.stop and actions.updateOptions, respectively.

For example, the following polls until there's a completed property in the response body:

new Polar({
    request: () => fetch("https://github.com"),
    onPoll: (response, actions, properties) => {
        if (response.body.completed) {
            actions.stop();
        }
    }
}).start();

In the next example, the endpoint is polled five times with the delay being doubled after each poll.

let delay = 1000;

new Polar({
    request: () => fetch("https://github.com"),
    delay,
    limit: 5,
    afterPoll: (actions, properties) => {
        delay *= 2;
        actions.updateOptions({ delay });
    }
}).start();

properties

This object contains two values.

properties.count gives the number of requests in the lifetime of the polling process.

properties.error contains the the error caught in the previous poll attempt, defaulting to null if there are none. This value is useful if you've set continueOnError to true and still want to track errors, otherwise any errors can just be caught in a catch block chained to Polar.start().

Note: the error value returned inside onPoll also refers to the previous poll attempt. This is because onPoll isn't called on error. It may be better to restrict error handling to the beforePoll and afterPoll methods to avoid confusion.

Stopping a poll from outside the process

It's possible to stop a poll outside the poll process by calling stop on the poll instance.

const p = new Polar({
    request: () => fetch("https://api.mysite.me")
});

p.start();

const anotherTask = () => {
    // Code for some other task here.
    // ...
    p.stop();
};