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# @joepie91/promise-delay-since
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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.
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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.
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## Example
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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.
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```js
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"use strict";
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const Promise = require("bluebird");
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const promiseDelaySince = require("@joepie91/promise-delay-since");
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let startTimestamp = Date.now();
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Promise.try(() => {
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// Logs immediately after running
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console.log("Hello world 1");
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return promiseDelaySince(startTimestamp, 2000);
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}).then(() => {
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// Logs 2 seconds after running
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console.log("Hello world 2");
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return promiseDelaySince(startTimestamp, 3000);
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}).then(() => {
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// Logs 3 seconds after running
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console.log("Hello world 3");
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});
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```
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## API
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### promiseDelaySince(sinceTimestamp, delay)
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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.
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- __sinceTimestamp:__ The reference timestamp to start counting from, as a 'UNIX timestamp' in milliseconds (like you get from `Date.now()`).
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- __delay:__ The delay as a number, in milliseconds.
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The Promise will never reject.
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"use strict";
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const Promise = require("bluebird");
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const promiseDelaySince = require("./");
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let startTimestamp = Date.now();
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Promise.try(() => {
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// Logs immediately after running
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console.log("Hello world 1");
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return promiseDelaySince(startTimestamp, 2000);
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}).then(() => {
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// Logs 2 seconds after running
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console.log("Hello world 2");
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return promiseDelaySince(startTimestamp, 3000);
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}).then(() => {
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// Logs 3 seconds after running
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console.log("Hello world 3");
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});
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"use strict";
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module.exports = function promiseDelaySince(previousTimestamp, delay) {
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return new Promise((resolve, reject) => {
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let targetTime = previousTimestamp + delay;
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let currentTime = Date.now();
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if (currentTime > targetTime) {
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resolve();
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} else {
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setTimeout(() => {
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resolve();
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}, targetTime - currentTime);
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}
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});
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};
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{
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"name": "@joepie91/promise-delay-since",
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"version": "1.0.0",
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"main": "index.js",
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"repository": "git@git.cryto.net:joepie91/node-promise-delay-since.git",
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"author": "Sven Slootweg <admin@cryto.net>",
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"license": "WTFPL OR CC0-1.0",
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"devDependencies": {
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"bluebird": "^3.5.4"
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}
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}
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