Add run-promises-in-series

snippets/run-promises-in-series.md

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+### Run promises in series
+
+Run an array of promises in series using `Array.reduce()` by creating a promise chain, where each promise returns the next promise when resolved.
+
+```js
+var series = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());
+// var delay = (d) => new Promise(r => setTimeout(r, d))
+// series([() => delay(1000), () => delay(2000)]) -> executes each promise sequentially, taking a total of 3 seconds to complete
+```