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javascript/snippets/promisify.md

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+---
+title: Promisify function
+type: snippet
+tags: [function,promise]
+cover: duck-plants
+dateModified: 2020-10-22T20:24:04+03:00
+---
+
+Converts an asynchronous function to return a promise.
+
+- Use currying to return a function returning a `Promise` that calls the original function.
+- Use the rest operator (`...`) to pass in all the parameters.
+- **Note:** In Node 8+, you can use [`util.promisify`](https://nodejs.org/api/util.html#util_util_promisify_original).
+
+```js
+const promisify = func => (...args) =>
+  new Promise((resolve, reject) =>
+    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
+  );
+```
+
+```js
+const delay = promisify((d, cb) => setTimeout(cb, d));
+delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s
+```