content(resources): add resource about converting a callback to a promise

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

@@ -0,0 +1,63 @@
+---
+authors:
+  - "veksen#1060"
+created_at: "2020/05/20"
+title: Converting a callback
+recommended_reading:
+  - javascript/promises/intro
+  - javascript/promises/async-await
+---
+
+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.
+
+Before anything check:
+
+- If your library might support both callbacks and API, in the documentation, or README from Github
+- If there is not another library that does the same thing, but supports promises.
+
+## From a callback
+
+Let's look at a conventional, but fictional callback API:
+
+```js
+getWeather("Los Angeles", (error, result) => {
+  if (err) throw err;
+
+  console.log(result); // "sunny"
+});
+```
+
+## To a promise
+
+If we want to use it as a promise, we'll need to wrap a promise around it:
+
+```js
+function getWeatherAsync(city) {
+  return new Promise((resolve, reject) => {
+    getWeather(city, (error, result) => {
+      if (err) reject(err);
+
+      resolve(result);
+    });
+  });
+}
+```
+
+Which we can now use:
+
+```js
+getWeatherAsync("Los Angeles")
+  .then((weather) => {
+    console.log(weather);
+  })
+  .catch((error) => {
+    console.log(error);
+  });
+
+// or using async/await
+const weather = await getWeatherAsync("Los Angeles").catch((error) => {
+  console.log(error);
+});
+
+console.log(weather);
+```