feat(resources) add util.promisify example

src/content/resources/language/javascript/promises/converting-a-callback.md

@@ -6,6 +6,9 @@ title: Converting a callback
 recommended_reading:
   - javascript/promises/intro
   - javascript/promises/async-await
+external_resources:
+  - text: util.promisify(original)
+    href: "https://nodejs.org/api/util.html#util_util_promisify_original"
 ---
 
 It's still pretty common to be forced to use an API that doesn't support promises. While a promise API is often preferred, it's not always available.
@@ -61,3 +64,19 @@ const weather = await getWeatherAsync("Los Angeles").catch((error) => {
 
 console.log(weather);
 ```
+
+In Node.js >v8.0.0 you can make use of the built-in `util` package which exposes a 
+[promisify](https://nodejs.org/api/util.html#util_util_promisify_original) method. This method helps us convert our original function to a promise-based function so that it returns the callback.
+
+Using our example from earlier, we can write it as:
+
+```js
+const util = require("util")
+
+const getWeatherAsync = util.promisify(getWeather)
+
+const weather = await getWeatherAync("Los Angeles").catch(console.log)
+console.log(weather)
+```
+
+**Note:** the `promisify` method adds a extra argument to the arguments you passed in. In this case we are calling the custom function with a single parameter of type `String`, which means the original function should also accept `(String, Function)`, `Function` being the callback `(error, result)`.