# @joepie91/promise-delay-since

A function that, when called, produces a Promise that resolves a specified amount of milliseconds since a particular timestamp - or immediately, if the delay since that timestamp has already passed.

This is particularly useful when implementing rate-limiting and/or task distribution code - your implementation only has to track the timestamp of the last call, and leave it up to this module to decide when to resolve a Promise that kickstarts the next task.

## Example

See also `example.js` for a runnable version. This example uses Bluebird for `Promise.try` (read [this article](http://cryto.net/~joepie91/blog/2016/05/11/what-is-promise-try-and-why-does-it-matter/) to understand why), but Bluebird is not required to use this module.

```js
"use strict";

const Promise = require("bluebird");
const promiseDelaySince = require("@joepie91/promise-delay-since");

let startTimestamp = Date.now();

Promise.try(() => {
	// Logs immediately after running
	console.log("Hello world 1");
	
	return promiseDelaySince(startTimestamp, 2000);
}).then(() => {
	// Logs 2 seconds after running
	console.log("Hello world 2");
	
	return promiseDelaySince(startTimestamp, 3000);
}).then(() => {
	// Logs 3 seconds after running
	console.log("Hello world 3");
});
```

## API

### promiseDelaySince(sinceTimestamp, delay)

Creates a new Promise that will resolve `delay` milliseconds from the specified `sinceTimestamp`. If the current time is later than `sinceTimestamp + delay`, the returned Promise will resolve immediately.

- __sinceTimestamp:__ The reference timestamp to start counting from, as a 'UNIX timestamp' in milliseconds (like you get from `Date.now()`).
- __delay:__ The delay as a number, in milliseconds.

The Promise will never reject.

## Changelog

### v1.0.1 (February 17, 2020)

- Fixed repository URL in package.json

### v1.0.0 (April 23, 2019)

Initial release.