# @joepie91/promise-delay-every

A function for writing *one per N amount of time* rate-limiting implementations.

When called, it returns a 'delayer' function. This delayer function can be called repeatedly; every time it is called, it returns a Promise that resolves `delay` milliseconds further into the future. The first time it is called, it resolves immediately.

So for example, if you specify a `delay` of 2000 milliseconds, the Promise returned from the first call will resolve immediately; the Promise returned from the second call will resolve after 2 seconds, the Promise resolved from the third call will resolve after 4 seconds, and so on.

All of these times are relative to the first call you make - so in the above example, there are 2 seconds between every resolution:

```
0ms     Call 1
0ms     Resolve 1
2ms     Call 2
3ms     Call 3
5ms     Call 4
2000ms  Resolve 2
4000ms  Resolve 3
6000ms  Resolve 4
```

If you wait longer than the delay to call the delayer function again, the returned Promise will resolve immediately and that time becomes the new base time for future delays:

```
0ms     Call 1
0ms     Resolve 1
2ms     Call 2
2000ms  Resolve 2
... time passes, beyond the 2000ms delay ...
7400ms  Call 3
7400ms  Resolve 3
7402ms  Call 4
9400ms  Resolve 4
```

## 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 promiseDelayEvery = require("@joepie91/promise-delay-every");

let delayer = promiseDelayEvery(2000);

return Promise.try(() => {
	return delayer();
}).then(() => {
	// Logs immediately after running
	console.log("Hello world 1");

	return delayer();
}).then(() => {
	// Logs 2 seconds after running
	console.log("Hello world 2");

	return delayer();
}).then(() => {
	// Logs 4 seconds after running
	console.log("Hello world 3");
});
```

## API

### promiseDelayEvery(delay)

Returns a new `delayer` function.

- __delay:__ The delay as a number, in millseconds.

### delayer()

Returns a new Promise that will resolve at the next scheduled interval, according to the algorithm described at the top of this documentation.

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.