promisify

Author: iliakan

1-js/11-async/07-promisify/article.md

@@ -0,0 +1,118 @@
+# Promisification
+
+Promisification -- is a long word for a simple transform. It's conversion of a function that accepts a callback into a function returning a promise.
+
+In other words, we create a wrapper-function that does the same, internally calling the original one, but returns a promise.
+
+Such transforms are often needed in real-life, as many functions and libraries are callback-based. But promises are more convenient. So it makes sense to promisify those.
+
+For instance, we have `loadScript(src, callback)` from the chapter <info:callbacks>.
+
+```js run
+function loadScript(src, callback) {
+  let script = document.createElement('script');
+  script.src = src;
+
+  script.onload = () => callback(null, script);
+  script.onerror = () => callback(new Error(`Script load error for ${src}`));
+
+  document.head.append(script);
+}
+
+// usage:
+// loadScript('path/script.js', (err, script) => {...})
+```
+
+Let's promisify it. The new `loadScriptPromise(src)` function will do the same, but accept only `src` (no callback) and return a promise.
+
+```js
+let loadScriptPromise = function(src) {
+  return new Promise((resolve, reject) => {
+    loadScript(src, (err, script) => {
+      if (err) reject(err)
+      elsee resolve(script);
+    });
+  })
+}
+
+// usage:
+// loadScriptPromise('path/script.js').then(...)
+```
+
+Now `loadScriptPromise` fits well in our promise-based code.
+
+As we can see, it delegates all the work to the original `loadScript`, providing its own callback that translates to promise `resolve/reject`.
+
+As we may need to promisify many functions, it makes sense to use a helper.
+
+That's actually very simple -- `promisify(f)` below takes a to-promisify function `f` and returns a wrapper function.
+
+That wrapper does the same as in the code above: returns a promise and passes the call to the original `f`, tracking the result in a custom callback:
+
+```js
+function promisify(f) {
+  return function (...args) { // return a wrapper-function
+    return new Promise((resolve, reject) => {
+      function callback(err, result) { // our custom callback for f
+        if (err) {
+          return reject(err);
+        } else {
+          resolve(result);
+        }
+      }
+
+      args.push(callback); // append our custom callback to the end of arguments
+
+      f.call(this, ...args); // call the original function
+    });
+  };
+};
+
+// usage:
+let loadScriptPromise = promisify(loadScript);
+loadScriptPromise(...).then(...);
+```
+
+Here we assume that the original function expects a callback with two arguments `(err, result)`. That's what we meet most often. Then our custom callbacks is exactly in the right format, and `promisify` works great for such case.
+
+But what is the original `f` expects a callback with more arguments `callback(err, res1, res2)`?
+
+Here's a modification of `promisify` that returns an array of multiple callback results:
+
+```js
+// promisify(f, true) to get array of results
+function promisify(f, manyArgs = false) {
+  return function (...args) {
+    return new Promise((resolve, reject) => {
+      function *!*callback(err, ...results*/!*) { // our custom callback for f
+        if (err) {
+          return reject(err);
+        } else {
+          // resolve with all callback results if manyArgs is specified
+          *!*resolve(manyArgs ? results : results[0]);*/!*
+        }
+      }
+
+      args.push(callback);
+
+      f.call(this, ...args);
+    });
+  };
+};
+
+// usage:
+f = promisify(f, true);
+f(...).then(err => ..., arrayOfResults => ...)
+```
+
+In some cases, `err` may be absent at all: `callback(result)`, or there's something exotic in the callback format, then we can promisify such functions manually.
+
+There are also modules with a bit more flexible promisification functions, e.g. [es6-promisify](https://github.com/digitaldesignlabs/es6-promisify). In Node.js, there's a built-in `util.promisify` function for that.
+
+```smart
+Promisification is a great approach, especially when you use `async/await` (see the next chapter), but not a total replacement for callbacks.
+
+Remember, a promise may have only one result, but a callback may technically be called many times.
+
+So promisification is only meant for functions that call the callback once. Furhter calls will be ignored.
+```

1-js/11-async/07-async-await/01-rewrite-async/solution.md renamed to 1-js/11-async/08-async-await/01-rewrite-async/solution.md

@@ -281,7 +281,7 @@ In case of an error, it propagates as usual: from the failed promise to `Promise
 
 ````
 
-## Async/await versus other async actions
+## Timiing: async/await and higher-level actions
 
 Some async stuff is more asynchronous than the other.
 
@@ -307,7 +307,7 @@ Remember promise queue from the chapter <info:promise-queue>? Promise `.then/cat
 
 `Async/await` is based on promises, so it uses the same promise queue internally.
 
-So `await` is guaranteed to work before any `setTimeout` or other event handlers. That's actually quite essential, as we know that our async/await will never be interrupted by other handlers or events.
+So `await` is guaranteed to work before any `setTimeout` or other event handlers. That's actually quite essential, as we know that our async/await code flow will never be interrupted by other handlers or events.
 
 ## Summary