Create promisify.md

snippets/promisify.md

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+### Promisify
+
+Creates a promise version of the given callback-style function. 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))
+    )
+// const stat = promisify(fs.stat)
+// When called, stat returns a promise
+```