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snippets/js/s/pipe-async-functions.md

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+---
+title: Pipe async functions
+type: snippet
+language: javascript
+tags: [function,promise]
+cover: new-york-skyline
+dateModified: 2020-10-22T20:24:04+03:00
+---
+
+Performs left-to-right function composition for asynchronous functions.
+
+- Use `Array.prototype.reduce()` and the spread operator (`...`) to perform function composition using `Promise.prototype.then()`.
+- The functions can return a combination of normal values, `Promise`s or be `async`, returning through `await`.
+- All functions must accept a single argument.
+
+```js
+const pipeAsyncFunctions = (...fns) =>
+  arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
+```
+
+```js
+const sum = pipeAsyncFunctions(
+  x => x + 1,
+  x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
+  x => x + 3,
+  async x => (await x) + 4
+);
+(async() => {
+  console.log(await sum(5)); // 15 (after one second)
+})();
+```